Twitter is a popular social-networking and messaging service, accessible through the web and mobile device apps, allowing users to write 140-character messages publicly or privately. Often the messages include hyperlinks, which get sent through URL shorteners (so they might suffer linkrot if the shortening services go away). Some of the conventions of the service are discussed in the article on Hashtags, at-signs, retweets, etc.
Of interest to archivers is the fact that, as of late 2012, Twitter has apparently started rolling out a feature to permit users to save their entire tweet history as an archive file. This is, as of now, not yet available to all users, but it supposedly turns up at the bottom of the "settings" screen in accounts in which the feature has been enabled.
More documentation on how to extract these archives and what format the files take will be supplied here once that information is available.