"SSH" gives you remote access to the command-line (like telnet), file transfers (a bit like FTP), and lets you run applications from a remote system on whatever Linux desktop you are on. Many Linux distributions include it by default, and it's very easy to work with.
This is a tutorial:
http://www.elsn.org/SSH-HOWTO/ssh-howto.html
It's a draft, so comments are welcome.
If you want to remotely access your Linux desktop from another system you'll probably want to use VNC, which displays the whole desktop as a window. This is a bit more fiddly to set up. There's notes here:
http://www.elsn.org/main/VncOnLinux
