A dictionary tells you what a word means. It gives you a "definition" for each word. When you want to know what a word means, you look in a dictionary.
A dictionary lists definitions or meanings.
A thesaurus gives you words that mean the same as, or nearly the same as, another word. When you can't find the exact word you want, you look in a thesaurus.
A thesaurus lists synonyms and similar words.
The plural of "thesaurus" is "thesauri" or "thesauruses".