No idea about windows.
ssh is encrypted, so the data which is sent accross the network is not visible to someone who can intercept the network traffic. The most pointed example of this is then you log into the network - with telnet, anyone who can listen to the network traffic can see your password (e.g. anyone who shares your LAN). With ssh, they cannot see your password.
Moreover, ssh offers lots of funky features which telnet does not. Important among these are public key authentication and port forwarding. RFCs are a good source of further information. Google for "rfc ssh" and "rfc telnet".