Two versions of the SSH protocol exist: SSH1 and SSH2. The two versions are different in terms of their underlying workings.
SSH1 is a simpler protocol than SSH2, and is available on a wider variety of platforms than SSH2. SSH1 has more options for authentication than SSH2, and performs better than SSH2.
On the down side, SSH1 has serious structural weaknesses that leave it vulnerable to a broad number of attacks. These weaknesses have been corrected in SSH2. Because the security enhancements in SSH2 would not have been possible if protocol-level compatibility with SSH1 had been retained, SSH1 and SSH2 are not compatible with one another.
SSH Communications Security, the original developer of the SSH protocol, considers the SSH1 protocol deprecated and recommends that SSH1 not be used.