"SFTP" can refer to either SSH file transfer protocol or Simple File Transfer Protocol. The first is a popular form of secure file transfer and the second is a now-defunct, slimmed-down version of FTP. SFTP should refer to SSH file transfer protocol specifically instead of secure FTP universally.
Unlike standard FTP, SSH file transfer protocol utilizes Secure Shell protocol to encrypt commands and data alike, protecting passwords and important information from being broadcast over a network. SSH file transfer protocol is not compatible with FTP software, so a server running one cannot operate with a client using the other. Embedded support that allows use of both protocols exists.