FTP is the Internet service used to transfer a data file from the disk of one computer to the disk of another, regardless of the operating system type.
Similar to other Internet applications, FTP uses the client-server approach — a user invokes an FTP program on the computer, instructs it to contact a remote computer, and then requests the transfer of one or more files. The local FTP program becomes a client that uses TCP to contact an FTP server program on the remote computer. Each time the user requests a file transfer, the client and the server programs cooperate to send a copy of the data across the Internet.
FTP servers which allow “anonymous FTP” permit any user, not only users with accounts on the host, to browse the “ftp” archives and download files. Some FTP servers are configured to allow users to upload files.
FTP is commonly used to retrieve information and obtain software stored in files at FTP archive sites throughout the world.
Source: Paraphrased from http://tlc.nlm.nih.gov/resources/tutorials/internetdistlrn/ftpdef.htm
See the manual: ftp.