software using the "File Transfer Protocol" (FTP).
FTP stands for "File Transfer Protocol". It is a standard network protocol for file transfer with a minimized overhead (and thus maximized throughput), requiring a server and a client component. While FTP itself is completely unsecured, it can be (and often is) secured with SSL/TLS (which then is called "FTPS"). On Unix systems, there's also SFTP – which sounds similar, but uses a different technology (ssh). For more details, please refer to the linked Wikipedia article.