SSH (Secure Shell) is a network protocol used to establish a secure connection between a client and server. Once a connection has been established, it acts like an encrypted tunnel down which data can be exchanged securely. It is used to maintain the confidentiality and integrity of data communications over insecure networks such as the Internet.
A SSH connection also provides password authentication using public-key cryptography. This form of cryptography works on a Private Key, Public Key system - the sending side has one of each, as does the receiving side of the transfer. Messages are encrypted with the recipient's public key and can only be decrypted with the corresponding private key.
Originally developed as a replacement for Telnet and other insecure remote shells SSH is used principally on Linux and Unix based systems to access shell accounts.
