The description on the website (I could write my own, but, you know, I'm lazy):
The Cook's Thesaurus is a cooking encyclopedia that covers thousands of ingredients and kitchen tools. Entries include pictures, descriptions, synonyms, pronunciations, and suggested substitutions.
Personally, I like to call it the "guaranteed to save your a$$ in the nick of time website". Because it has done just that for me, on more than one occasion. :)