A dictionary is a type of book which explains the meanings of words or, more precisely, lexemes. The words are arranged in alphabetical order so that they can be found quickly. The word "dictionary" comes from the Latin "dictio" ("saying").
There are several types of dictionaries: dictionaries which explain words and how they are used, dictionaries which translate words from one language to another, dictionaries of biography which tell about famous people, technical dictionaries which explain the meanings of technical words or words connected to a particular subject (sometimes called a thesaurus). Some of these come close to being an encyclopedia, but an encyclopedia gives a lot of extra information about things (knowledge) and does not explain the use of the language. An encyclopedic dictionary gives less information about the topic than a real encyclopedia does, but more than a simple dictionary.