FTP or file transfer protocol is used to connect two computers over the Internet so that the user of one computer can transfer files and perform file commands on the other computer.
Specifically, FTP is a commonly used protocol for exchanging files over any network that supports the TCP/IP protocol (such as the Internet or an intranet). There are two computers involved in an FTP transfer: a server and a client. The FTP server, running FTP server software, listens on the network for connection requests from other computers. The client computer, running FTP client software, initiates a connection to the server. Once connected, the client can do a number of file manipulation operations such as uploading files to the server, download files from the server, rename or delete files on the server and so on. Any software company or individual programmer is able to create FTP server or client software because the protocol is an open standard. Virtually every computer platform supports the FTP protocol.
For the rest of the Wikipedia entry on the above term, go here.
Filezilla is a great FTP (File Transfer Protocol) program, and it's free. While this movie is intended for home inspectors, it is useful for anyone who maintains a web site.