SSH (Secure SHell) is a protocol for remotely logging into a machine. Much like Telnet, an SSH client will provide the user with a text session ("shell") on the remote machine. Unlike Telnet, SSH provides an encrypted connection between the client and the server. In practical terms, the difference between using Telnet and SSH is much like the difference between visiting ordinary websites and visiting so-called "secure" websites.
Similarly, SFTP (Secure FTP) is a replacement for FTP which provides an encrypted connection between the client and server for file transfer.