There are different autopilots for planes. They differ in capabilities. Mostly the pilot sets a heading, altitude and speed and the airplane will maintain that. The main collision avoidance system is Air Traffic Control telling the pilot where to fly when. Then there are other warning systems that only alert the pilot (terrain and other planes).

The real autopilot in planes is called Auto Flight System and is very recent and needs to be monitored closely. It can lift off, fly and land on it's own, but basically only in good weather condition.