FTP is the acronym for File Transfer Protocol. It is the set of instructions and the rules for transferring files from one place to another over a network. It is particularly useful when the number of files is large, such as when uploading a web site to the World Wide Web.
Originally, and for the hardy, FTP was used on a terminal by typing in characters to the FTP application. Instructions such as dir, get and put did the obvious things. For ease of use, there are now windows based interfaces which show the folder or directory structure of the source and destination machines, so it is possible to use it without knowing the protocol.