File Transfer Protocol is a standard network protocol used to transfer data across TCP-based networks between hosts.
File Transfer Protocol (Wikipedia article) is a protocol, governed by well-established Internet standards, for conducting file operations (e.g. transferring files between two systems) over a TCP-based network such as the Internet or a local area network. FTP employs a client-server architecture, where one system is the "client", which opens a connection to the other system, the "server", the client makes requests of the server, and then closes the connection.
The most common FTP operations that a client will initiate include downloading files (i.e. "get" files from the server) and uploading files (i.e. "put" files on the server). There are a variety of file system related commands that the client request of the server (e.g. making directories, renaming files on the server, deleting files from the server, etc.).