FTP (File Transfer Protocol)
The FTP protocol has been developed 1985 to transfer files over the IP protocol. The default FTP port is 21.
Main problem with FTP nowadays is the the fact, that the authentication is NOT ENCRYPTED and can therefore easily be captured if your computer is in the same network as someone with a running Packet-Sniffer like WireShark.
Therefore it is not recommended!
FTP with implicit SSL
FTP with implicit SSL is the next iteration of FTP which should fix the main problem – no encryption. Data transfer usually happens over the Port 990 but before sending the login credentials and further commands a SSL or TLS connection is being established (dependent on the server configuration). The base of the FTP prototoll still stays the same!
Dependent on the server configuration (mainly the used encryption method) you can use this method in a live/production environment.
FTP with explicit TLS
FTP with explicit TLS is more “flexible” than FTP with implicit SSL. First of all the connection is established via the default FTP port 21, but the client has the choice if it wants to just transfer the login data or everything over the TLS connection.
Main problem here is the encryption certificate needed. A valid, signed certificate for this transfer method can only be acquired via a certificate authority (which usually means you have to spend money). You can of course use a self-signed certificate, but on every client connecting to your sever there will be a warning that the used certificate is not signed.
In comparison SFTP doesn’t need any certificate since everything is handled via the SSH protocol.
The SSH File Transfer Protocol doesn’t have anything in common with the old FTP protocol since it is based on the SSH protocol and all commands are being sent over one, encrypted connection.
Therefore this is currently the recommended way to transfer data from one PC to another via any network. Also its pretty easy to setup since the SFTP subsystem is present in the default installed SSH-Daemon on a linux system. You just have to activate it.
In comparison for any FTP connection you always have to (no matter if encrypted or not) install and configure a separate FTP server like z.B. VSFTP or ProFTP.
rsync is a program which is also based on the SSH protocol like SFTP. The main difference is, that only files are being transferred which have really been changed.
See HERE for all details of rsync.