A thesaurus is a dictionary of synonyms and homonyms. Synonyms are words having similar meanings. A dictionary, on the other hand, gives definitions and pronunciations. Sometimes a thesaurus gives antonyms as well.
Different words with very similar or identical meanings are called synonyms. The words beast and animal are synonyms because they have very similar meanings. Similarly, the words woods and forest are synonyms. Words that are synonyms are said to be synonymous.