You install a small program on your PC and then download the app to your phone. Then, even through 3g, you can connect and use your PC from your phone as if you were right there. You can even see through any webcam you have setup on the PC at home.
When I show people my phone and they see my Windows 7 desktop on the screen, they are amazed.
Logmein does one similar but it's way too expensive at 29.99 This one is only 9.99 (at least while it's still in beta) so I bought it for cheap while I could.