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).