A thesaurus is a reference book (or a really cool website, like the Visual Thesaurus) with an organized list of words and their synonyms.
An Englishman named Peter Roget published the first modern English thesaurus in 1852. Roget had severe depression, and one of the ways he coped was to put his list-making and lexicographical (word analyzing) skills to use in a reference book that would group words, not alphabetically, but according to how similar their meanings were. This being the middle of the 19th century, there were no computers: Roget compiled and sorted all of his lists by hand.
