SSH Protocol Overview:

SSH is the abbreviation of The Secure SHell. SSH operates over TCP connections, usually to port 22. Secure Shell is a program to log into another computer over a network,  to execute commands in a remote machine, and to move files from one machine to another. It provides strong authentication and secure communications over insecure channels. It is a replacement for telnet, rlogin, rsh, rcp.

SSH protects a network from attacks such as IP spoofing, IP source routing, and DNS spoofing. An attacker who has managed to take over a network can only force ssh to disconnect. He or she cannot play back the traffic or hijack the connection when encryption is enabled.

The SSH-2 protocol is described in the following five main documents.

SSH Protocol Architecture
     Describes the overall design of SSH-2.

SSH Transport Layer Protocol
     Provides a single, full-duplex, byte-oriented connection between client and server, with privacy, integrity, server authentication, and man-in-the-middle protection.

SSH Authentication Protocol
     Identifies the client to the server.

SSH Connection Protocol
     Provides richer, application-support services over the transport pipe, such as channel multiplexing, flow control, remote program execution, signal propagation, connection forwarding, etc.

SSH Assigned Numbers
     Gathers together and lists various constant assignments made in the other drafts.

References:

What's the difference between versions 1 and 2 of the SSH protocol?