Dictionary

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.
Dictionaries which explain the meaning of words
Online dictionaries
TheFreeDictionary
Longman English Dictionary Online
Dictionaries which explain what words mean will give a clear "definition" of the word (e.g. hippopotamus : a hoofed mammal with thick skin, large mouth and short legs that lives in rivers and lakes of Africa.)
A big dictionary will also give more information about the word. It will explain how it is pronounced. Usually the International Phonetic Alphabet is used for this. It will explain how the word is used. This is not a problem for a word like "hippopotamus", but a word like "put" has so many different meanings that a large dictionary may have a whole page or more to explain how it can be used. It will also explain the origin of the word (e.g. Greek "hippos" horse and "potamus" river).
A dictionary may also give the form of the word in different tenses, plural form etc.
Dictionaries which translate into foreign languages
There are also dictionaries which translate words into foreign languages. Often one volume (one book) will translate both ways; for example, half the book might be translating from English to Dutch and the other half from Dutch to English.
When using a dictionary to find out how to say something in another language one has to be careful to choose the right word. A word like "right" has two basic meanings in English: 1) "correct", and 2) the opposite of "left". Other languages have different words for these different meanings, but they have homonyms of their own. A word like "put" has many meanings. A good dictionary will have a large list of these meanings to help people find the word they want. In many languages, for example, the word "put" will be different according to whether something is being put onto something (e.g. a table) or into something (e.g. a cupboard).
Updating dictionaries
Dictionaries need to be updated frequently because of the way language changes. New words are often brought into a language (e.g. lots of computer terms) or words change their meanings (e.g. "gay" or "cool"). In this sense, the most famous English Dictionary is the Oxford English Dictionary (or OED). Words are always being added to the OED. They are never taken out even if they are obsolete (not used any more). The OED can be accessed online (with a subscription).