A significant problem with "plain" FTP is that it is not secure - usernames, passwords and data are sent across the network in the clear. This means eavesdroppers sniffing the network will have little trouble obtaining credentials and copies of the transferred files.
FTPS was designed to fix this by encrypting communications using the SSL/TLS protocol, which is specifically designed for securing network connections.
FTPS is an IETF standard as described in RFC 4217.
See also:
· Implicit FTPS and Explicit FTPS
· Securing Control and Data Channels
· FTPS Commands
· FTPS Usage