These commands allow access to a remote machine. rlogin and telnet allow you to login remotely to another UNIX machine. The rsh command allows you to start a shell or execute a command on the remote system.
The difference between rlogin and telnet is that if you have an appropriately configured .rhosts file on the remote system, you do not have to specify a username and password to login using rlogin. With the telnet command you always have to specify both a username and a password to login. You need to have an .rhosts file on the remote system to use the rsh command on the remote system to execute a command. For more information on configuring an .rhosts file type man rhosts.