What exactly is a head gasket on a car?

I just got a 97 Saturn coupe and the original owner told me that the only problem with it is a blown head gasket. I want to know if that is something that I can fix as sort of a summer project or if I'll have to have a mechanic repair it. The car started up, and I didn't hear anything funny with the engine, but just noticed some smoke coming out of the muffler and that the car started sputtering after about 2 minutes of running, almost as if it was about to die. Also where can can I find a trunk latch since the trunk is not closing properly and also where to find the interior roofing??