BOINC (Berkeley Open Infrastructure for Network Computing) is the new distributed computing infrastructure for SETI@Home (Search for Extra Terrestrial Intelligence).
The current version of SETI@Home is winding down and the next version is based on BOINC. As BOINC is an open infrastructure, unlike SETI@Home Classic, several other projects besides SETI@Home are also using BOINC.
Currently BOINC is still in testing so you may experience problems running this version of SETI@Home. Particularly, Berkeley have had problems with their servers and project database. If you are unsure BOINC is currently for you, then you can always stick with SETI@Home Classic until all the current issues have been resolved.