One of the earliest methods of Internet publication, anonymous FTP uses the FTP Protocol along with some simple conventions.
FTP was designed to let a user connect a remote system on which he had an account, authenticate himself using a userid/password combination, then navigate a directory hierarchy and retrieve files.
Anonymous FTP extends this idea by allowing users without accounts to use FTP for retrieving "public" data. To do this, a user connects to an anonymous FTP server with a normal FTP client, offering anonymous as a userid and sending an identifying string, typically an email address, as password. Servers configured for anonymous FTP will accept almost anything as password, so this information is really based on an honor code.
Once connected in this manner, the user can examine the server's file repository and download anything of interest using FTP's standard capabilities. Anonymous FTP servers typically implement various security measures to prevent anonymous users from access anything but an area designated for public information.
Anonymous FTP sites, like web sites, proliferated everywhere in near-total anarchy. Just as tools such as Yahoo and WebCrawler are used to index web sites, a program called archie indexes anonymous FTP sites. Archie is not free software, though it's client implementations are free and widely available. Among these are Web interfaces such as ArchiePlex.