Horseradish is a member of the mustard family (sharing lineage with its gentler cousins, kale, cauliflower, Brussel sprouts and the common radish) and is cultivated for its thick, fleshy white roots.
The “hotness” from horseradish comes from isothiocyanate, a volatile compound that, when oxidized by air and saliva, generates the “heat” that some people claim clears out their sinuses.
The bite and aroma of the horseradish root are almost absent until it is grated or ground. During this process, as the root cells are crushed, isothiocyanates are released. Vinegar stops this reaction and stabilizes the flavor. For milder horseradish, vinegar is added immediately.
See Also
Is Horseradish a Radish?