Same general idea - a client downloads packets of information to process, runs on the cycles you aren't using, and uploads the results. You're donating cycles to find cancer cures - a worthwhile cause.
Aliens are really cool.
UD has no aliens. The user interface of what you download is much better. SETI runs when in screensaver mode, or if the window is open. UD has an additional option of allowing it to run all the time, even with the window minimized. You can customize to have it run only at certain times on certain days.