Tempeh is the result of the fermentation of cooked soybeans by a Rhizopus mold. It originated in Indonesia, where written sources mention it in the 16th century, although it's probably much older (it's similar to Chinese soybean ferments). A staple source of protein there, tempeh didn't really hit the U.S. until the 1960s, where, until recently, it remained the purview of hippie communes. I was interested to read (on this site) that efforts have been made to introduce tempeh to developing countries as a cheap source of protein, but they have mostly failed in areas where there is no local tradition of mold-fermented foods.
I like tempeh's texture better than tofu's, and I've heard rumors that fermented soy products are better for you. Another plus is that tempeh is less processed and uses more of the actual soybeans (whereas tofu is produced from soymilk). I don't mean to knock tofu, it's just not as life-changing.
If you're interested in turning soybeans into soymagic, read on...