A dictionary, by definition is a book or electronic resource that lists the words of a language, typically in alphabetical order and states their meaning and often provides information about pronunciation, roots, origin and usage. On the other hand a thesaurus is a book that lists words in groups of synonyms and related concepts. A dictionary is often used to find how to spell a word or what the definition of the word is, a thesaurus is used to find synonym and antonyms.
(I don't know if its 50 words or not, but I hope it helps!)
