Since the plant is difficult to grow, most wasabi sold outside of Japan is just European horseradish with green coloring added.
Wasabi (Japanese: 山葵 or 和佐比; scientific name Wasabia japonica (syn. Cochlearia wasabi, Eutrema japonica)) is a member of the cabbage family. Commonly known as Japanese horseradish, it grows naturally along stream beds in mountain river valleys in Japan. It is green and has an extremely strong flavor. Its hotness is different from chili pepper, which burns the tongue; wasabi's strong sensations shoot up one's sinus cavity instead.
The historical purpose for wasabi is supposedly to kill the bacteria in the raw fish often used in sushi.