A thesaurus differs from a dictionary because a thesaurus?

gives only a short definition.

A thesaurus provides synonyms (similar words) as well as a definition of a word.

No, a thesaurus is not the same as a dictionary. A dictionary gives the definitions of words and a thesaurus gives synonyms of words.

Both dictionaries and thesauruses are useful, and they are both good when writing reports. Even though they both have different uses, they are, in a way, the same. :)