Amateur Radio Emergency Services, or ARES, is the field arm of the Amateur Radio Relay League (ARRL). The League deals with all aspects of Amateur Radio, including legislation, licensing, and contests; the ARES branch specifically handles field communications, particularly during emergencies. When you hear in the news that Amateur Radio operators were part of a search and rescue operation, assisted in getting aid to a ship in trouble at sea, or provided communications for a Red Cross shelter, you're hearing about an ARES function.
ARES volunteers also offer a splendid community service by providing free administrative communications at planned events, such as air shows, parades, and bike rides. In addition to helping the community through such service, experienced and inexperienced operators alike can use such events to refresh and polish their emergency-response skills. (To find out more about upcoming planned events in Santa Clara County, check out the connection for Training.)
ARES operators provide their own insurance.
You become an ARES amateur radio operator by joining ARRL, but a "Ham" does not need to be a member of ARRL to participate in ARES. If you become an Emergency Coordinator (EC), you do need to join ARRL.