Psilocybin Truffles vs. Psilocybin Mushrooms

Xavier Francuski, writing for The Third Wave:

Psilocybin Truffles and Psilocybin Mushrooms are very similar psychedelics, in that they contain the same active compounds (psilocybin, psilocin and baeocystin), but differ in how they are produced. Psilocybin Truffles are underground clumps formed out of mycelial strands, whereas Psilocybin Mushrooms grow out fully when conditions are favorable.

Even though they are made of the same stuff, there are certain distinctions between them. 

An excellent breakdown of the differences between truffles and mushrooms, including variations in legality, trip intensity, perceptual characteristics, and safety.