FTP is a file transfer protocol that does not include any options for encrypting data in transit. It was originally designed for use in private scientific and research networks and is based on a specification defined in 1985 by the Internet Engineering Task Force in RFC 959. FTP uses two connections to send data. Authentication data (e.g., usernames and passwords) is exchanged on a command channel. Data files are sent on a separate channel that is established after the authentication is complete.
Secure FTP is a broad term that refers to two different technologies that can encrypt both authentication information and data files in transit.
Secure FTP protocols protect data only while it is being transmitted. Once data files have been written to a secure FTP server, the data is no longer protected unless the files were encrypted before transmission. A typical scenario is to encrypt files using a tool like PGP and then transmit using either SFTP or FTPS.