|Truffles are hypogeous (underground) versions of mushrooms. They don't form a prominent stem and their spore-bearing surfaces are enclosed. They rely on animals eating them (mycophagy) to distribute their spores, instead of air currents like mushrooms. Truffles resemble small potatoes, and often between the size of a marble and a golf ball. There are hundreds of different kinds of truffles, and while none are known to be poisonous, only a few of them are considered to be delicacies by humans.
In Europe, the term "truffles" in the very strictest ("true") sense has historically only referred to those hypogeous (belowground) fungi that were gourmet edibles, primarily in the genus Tuber. Tubers are members of a larger grouping of fungi called Ascomycetes. For some folks, only Tubers and their close relatives are considered "true truffles" (all others being "false truffles"), and for other folks all hypogeous Ascomycetes (irrespective of culinary qualities) are called "true truffles". However, there are many, many other perfectly nice species of hypogeous fungi, both Ascomycetes and Basidiomycetes, that are not particularly prized as food outside the squirrel community. Nowadays many folks have discarded the "true truffle" and "false truffle" distinctions for a more egalitarian nomenclature, and are quite comfortable simply calling all hypogeous fungi TRUFFLES.
Are any truffles poisonous?
No truffles are known to be poisonous to humans (but we don't know everything...). This non-toxicity seems sensible, considering that truffles rely on small animals (via mycophagy) to distribute their spores. That said, ALWAYS be absolutely sure of the identification of anything you are considering eating! Many poisonous Amanita and Cortinarius mushrooms start out as belowground "eggs" that can be dead-ringers for truffles at a glance. The Oregon white truffles that most folks around here are interested in eating (Tuber gibbosum & T. oregonense) are firm and brittle, will have a beige to smokey-colored marbled interior, and a pleasant earthy odor. Mushroom eggs (and many truffle species that are only appetizing to squirrels) are generally more squishy, spongy, or cartilaginous - always check every truffle you intend to eat, different species frequently intermix.
Where are truffles found?
Truffles can potentially be found almost anywhere there are trees. Only a few families of trees (such as maples and cedars) do not associate with truffle-forming fungi. Truffles fruit throughout the fall, winter, and spring, depending on species and locality. They usually occur at the interface between the organic litter and the mineral soil, about one to six inches deep, but can emerge to the surface or be more than a foot deep. Evidence that small animals have been digging in an area recently is often a good indication that truffles may be about.