Dictionaries and thesauruses are both reference books for words. A dictionary explains the meaning of the word, as well as showing how the word is spelled and employed. A thesaurus is a book that includes words that are different but mean the same as the original word.
These are called synonyms. Thesauruses are very helpful for those in business, especially writers. Having a book to demonstrate that there is another word for the original word is a Godsend to those whose profession is writing. There are several types of dictionaries.
Some use the International Phonetic Alphabet, while others use the alphabet to display pronunciations. Most dictionaries will also explain the different usages and parts of speech. There are also dictionaries for specialized subjects, such as medicine and geography.