Wasabi is Japanese horseradish. It is green, and EXTREMELY strong, to the point that more than one person is quoted as saying it's "Hotter than Hell", although most just gag and wheeze their way to the nearest source of liquid. Its hotness is different from chili pepper which burns the tongue, wasabi's strong sensation shoot up one's sinus cavity instead.
It is sold either in root form, dried powder which mixed with water to make a paste or as a ready to use paste, which comes in tubes approximately the size and shape of travel toothpaste tubes.
Thankfully for those either through malice or unfamiliarity come into contact with too much of this condiment, the burning sensations it can induce are shortlived compared to chillis.
Most of the "wasabi" served today is really just really strong horseradish dyed green. The demand for real wasabi is very high -- in fact, the state of Oregon is one of the world's top producers of this condiment.