ssh is a secure version of the standard rsh. When connecting to machines, you should always use ssh. Forget about telnet, rsh, rlogin, rcp, and the like and use their secure analogs, ssh, slogin, and scp instead.
There are numerous advantages, but the biggest one is that ssh encrypts the connection that it creates, so people on the network sniffing for passwords won't see yours in plain text like they would if you were using telnet. ssh also verifies that you're connecting to the machine that you think you are.
ssh also automatically forwards X windows connections over the encrypted connection. If you ssh from machine A into machine B and start an X application, it will automatically be displayed over the secure connection on machine A.
Free versions of ssh are available for Windows and UNIX machines. See the FAQ page at http://www.cs.umd.edu/faq for more details.
See also the man page for ssh.