To participate in SETI@home, you must give an address where you receive email. This address will not be shown on the SETI@home web site or shared with organizations. Private messages sent on the SETI@home web site are visible only to the sender and recipient. The screensaver downloads a packet of data containing a work unit of radio signals and then grinds away on them. With the SETI@home screensaver installed, your computer actually processes SETI data when it is idle. Please remember to be considerate of other members. SETI@home may send you periodic newsletters; however, you can opt out at any time. Reply to: SETI@home - Is It Safe - or Spyware PLEASE NOTE: Do not post advertisements, offensive materials, profanity, or personal attacks.
So SETI@home created its own screensaver, which you install on your machine.