Boletes look like regular gilled mushrooms in that they have a cap and a stem, but they have a soft fleshy pore surface under their cap instead of gills. The pore layer is easily separated from the flesh of the cap, and the whole mushroom will feel soft enough to easily chew. Other unrelated mushrooms called polypores may have a pore surface, but they are usually tougher - usually at least as tough as leather if not as hard as a chunk of wood, and the pores cannot usually be easily removed from the rest of the mushroom (with the possible exception of Boletopsis). Boletus usually grow out of the ground, while polypores usually grow out of the side of trees without stems, or are simply a pored surface lying flat without a cap on a piece of wood, but occasionally they will have the shape of a regular mushroom.
Most boletes are mycorrhizal. Their spores are often three times or so longer than they are wide.
Like most non-gilled mushrooms, boletes are generally safer to eat and easier to identify than gilled mushrooms, although there are important exceptions. Gilled mushrooms have the greatest diversity and for whatever reason, more toxic species. In fact, some of the most desired edibles are boletes, in particular the porcini or king bolete (the Boletus edulis group) and many of the scaber-stalked boletes (Leccinum - although some Leccinums cause allergic reactions in some people).
Poisonous boletes are rare in the PNW. The most reliable reports of severe bolete poisonings come from those with red pores, like Boletus satanas (Rubroboletus eastwoodiae). Others, like Boletus luridiformis are not poisonous. A friend ate one a couple years ago because he suspected that all red pored boletes were unfairly getting a bad reputation, and sure enough, it turned out to not be poisonous (at least to him).
There are some quite common boletes that taste very bitter or very hot. These have been reported poisonous, because people eating them sometimes got an upset stomach, but I don't know if that's because of the unpleasant taste or if they're actually poisonous.
Key to Boletes: (the larger ones maygrow to >20cm)
There are many different kinds of boletes. We don't have very many of them in the PNW compared to other parts of the world like the east coast, where it is joked that they have more genera than we do species. You should be grateful you don't have to learn them. Boletus is the Latin genus that most people think of when they think of the word bolete (as you might guess because the words are very close), but Boletus (capitalized) is just one kind of bolete (not capitalized). Many yummy species are here.
The king boletes: once the DNA work is complete, it is likely that these are going to be the only species still called Boletus. Everything else will get new genera. (Changing some of the genera, depending on your point of view, may be complicating things unnecessarily. For a bunch of species that are all each other's closest relatives, it's kind of a matter of opinion if they are "different enough" to justify making everybody learn a bunch of new names). Mushrooms in this main group are often called the porcini or cep. They can be huge (up to 30cm) and are highly prized. There are many species in this complex, all equally tasty.
All have a net-like reticulation pattern on the top of a (usually) bulbous stem. Pore surface white when young, yellow when mature. They do not usually turn blue, but sometimes slightly so.
B. edulis - "King", brown cap, whitish young pores (most others are yellowish) and bulbous stem. One variety (var. grandedulis) is even larger and has reddish tones in age.
B. mottiae - has a strongly reticulated cap surface.
B. regineus (aereus) - "Queen", usually darker capped with a white bloom, with oak (although sometimes found near conifers). Often squat. Gastroboletus subalpinus is related.
B. rex-veris - "Spring King", yellower pores than the others, reddish brown on cap and on stem.
B. 'pinophilus' - fall version of the same species, perhaps less club shaped stem, like B. fibrillosus, but with a smoother cap. (KOH negative).
Butyriboletus - the butter boletes: like the king boletes with reticulated stems and often bulbous stem bases, but with yellow pores, stem and flesh that often turn blue. Large and stocky (10-15cm or so). They get rarer the further north you travel from California. The European species B. appendiculatus (not red) and B. regius (red) are actually the following PNW species:
Caloboletus - the bitter boletes: May be huge (20-30cm), very common. Mistake these for porcini and you'll wonder what all the fuss is about, but these all blue, and don't have the bulbous stem often seen in B. edulis. Not to mention that they are usually horribly bitter.
C. calopus (frustosus) - large and bitter tasting, rough cap, reticulated stem top, red stem bottom, not bulbous, bluing.
Rubroboletus/Neoboletus/Suillellus - the red-pored boletes, some of which can be quite poisonous (at least Rubroboletus is). All are rare, and turn blue. Usually 10-20cm.
Rubroboletus haematinus - at first with only hints of red in the pores, watch out! Otherwise similar clavate, reticulated species.
Aureoboletus/Buchwaldoboletus - two distinctive species.
Aureoboletus flaviporus - unusual viscid cap and extremely vivid yellow pores. Hardwoods, especially madrone.
Chalciporus - smaller (<10cm), hot and peppery, yellow to brown pored boletes. Yellow mycelium at the stem base. Perhaps they are parasitic on Amanita muscaria, but it's hard to tell if they are restricted to growing near that Amanita since that grows almost everywhere.
Tylopilus pseudoscaber (Porphyrellus porphyrosporus) - a rare greyish-black pored bolete. The east coast has lots more species, and they don't all have the dark pores that make them easy to spot. When young the pores might be whitish, but the rest of the mushroom is still dark. Blues, the blue quickly turning brown. 5-15cm.
'Boletus' mirabilis - the Admiral. Very velvety cap with a dark stem streaked like a tree trunk (compare B. fibrillosus) but has yellow pores and grows on wood. It tastes of butter and lemon! Rarely blues. Up to 15cm or more.
Xerocomus and Xerocomellus: usually not reticulated. Usually small-medium (<10cm).
Xerocomellus chrysenteron grp - diffractus/
Xerocomellus zelleri grp - very tasty bite sized morsels. Beautiful black cap contrasted with yellow pores and red stem. Some may blue slightly. X. zelleri has a smoother, shinier cap with a pale rim. X. atropurpureus is more common.
Other smaller (5-15cm), not reticulated boletes.
'Boletus' smithii - the top of the stem is red (unlike B. rubripes), and the brownish yellow cap becomes more red as you rub it. Blues.
'Boletus' coccyginus - stem is less red than in B. rubellus. Probably introduced, found with cottonwoods and non-native trees. Does not usually blue.
'Boletus' rubellus - Red cap and stem contrasted with bright yellow pores and stem top. May blue. Rare and introduced. Perhaps a Hortiboletus.
Gyroporus 'castaneus' - a rare and interesting small (5cm or so) non-native Boletus relative that has been introduced under landscaped hardwoods. It almost looks like it has notched or free white pores, a concept normally only used for gilled mushrooms. Does not blue.
Leccinum: the scaber-stalked boletes, not to be
confused with Suillus which has glandular dots that look painted on compared to
scabers that can be removed. These are edible, although some people report
allergic reactions to the red capped ones (which stain blue/
L. scabrum - the birch bolete, often found in your yard with your birch tree. Dark brown cap. May blue in stem base.
L. brunneum - dark brown capped aspen bolete. Idaho.
L. holopus - an albino birch bolete with a white cap. May stain red.
L. rotundifoliae - paler than L. scabrum but not usually white. Birch. Idaho.
L. clavatum - an albino conifer Leccinum with a thick, clavate stem. Stains blue-grey. Large. Idaho.
The smooth orange-red capped group is confusing and not well studied yet, but here is what others have said so far. Note that only one group has black scabers when young, the rest darken in age, possibly to black. I doubt these species concepts will hold up to DNA scrutiny. If I was a betting man, I'd bet that the species you find might have most to do with what trees are nearby.
Suillus - The Slippery Jacks, considered inferior edibles to most of the other genera, although some are said to be good. Suillus is suggested by a sticky cap instead of a dry one, or a veil, or glandular dots on the stem (but often all three). Suillus tomentosus has no veil and sometimes a dry cap, and therefore is probably the most difficult one to recognize to genus. Many of them have a preference for pine trees. The glandular dots are simply coloured spots that look painted on, unlike the scabers of Leccinum which can be removed. Some of them may stain somewhat blue in the pores or flesh when handled or exposed to oxygen, while others turn brownish. 5-10cm or more unless otherwise specified.
Let's start with the ones that do not have a veil. It can be tricky to tell... often the veil might rub off in old age so it's easiest to tell on a young specimen, but if you look carefully you can usually tell because there will be some kind of clue or remnant left around the stem or cap margin. It just takes practice to know what to look for. These do not blue, except for S. tomentosus.
Suillus brevipes - dark cap, no dots on the stem (thus a clean look), often a short stem, usually under pine. 2-needle pine.
S. pallidiceps - an almost albino version from ID.
S. weaverae ('granulatus') - like S. brevipes, but usually paler with glandular dots on the stem. 5-needle pine.
S. tomentosus - rough, almost dry cap and no ring, orange colours, including in the pores. Slowly blues. 2-needle pine.
S. discolor - less yellow cap. 5-needle pine.
Rough-capped Suillus: These do not appear very Suillus-like because the caps are not very sticky, in fact they are pretty rough and scaly, but they do have an obvious veil. Some exhibit a slight bluing: S. lakei in the base of the stem and S. ochraceoroseus throughout.
S. lakei - a pinkish brown distinctively scaly cap with large pores that stain brown. One version is very much like S. caerulescens with subtle scales. Douglas fir.
S. ampliporus ('cavipes') - dark brown with large pores. Easily recognized by its hollow stem. Larch.
These species have a weak partial veil, that usually does not stay on the stem to form a ring, you may only see some remnants along the edge of the cap. They all have at least some conspicuous glandular dots on the stem. They do not blue.
S. brunnescens (borealis) - young white cap and stem staining chocolate brown, weak purple- to red-brown veil, with stem dots. 5-needle pine.
S. glandulosipes (neoalbidipes) - like S. granulatus, but with more dots and a veil on the young cap (and possibly stem). 2-needle pine.
Next, species with both a sticky cap and ring, and also with many, conspicuous glandular dots on the stem.
S. luteus - the Slippery Jack itself. Dark cap colour and purple sheathing veil (splays up). Lots of glandular dots. S. borealis can sometimes look similar. 2-needle pine.
Finally, the sometimes difficult to recognize species without conspicuous glandular dots on the stem:
S. caerulescens (imitatus var. imitatus) - streaky yellow brown cap. Gills bruise brown and only the bottom of the stem bruises blue. Suillus lakei can look very similar with sometimes subtle scales. Douglas fir.
Suillus ponderosus (imitatus var. viridescens) - a little darker brown than S. caerulescens, occasional green patches, often bigger and stockier, with yellow-orange slime on the veil when young. Douglas fir.
S. clintonianus ('grevillei') - usually chestnut coloured with a yellow rim and a yellow to chestnut veil, but harder to recognize when the colours fade. Also staining brown, but sometimes blue in the stem base. Larch.
S. elbensis ('viscidus'/
S. flavidus (umbonatus/
S. pseudobrevipes - often short stem, small sheathing veil (splays up). Not bluing. 2-needle pine.
S. albivelatus - with a white veil, like so many others, but the veil not sheathing like in S. pseudobrevipes, and it may leave material on the cap, not just at the edge. Not bluing. 2-needle pine.
Congratulations! You are now familiar with an entire group of mushrooms that contains many good edibles, all of our fleshy pored mushrooms. For specialized literature, please see North American Boletes by the Bessettes et. al.