[Fasterthanyou]

There are a few different things that can happen.
What Justin was talking about is one instance but I've found the more popular seize to actually be that of the rings in the bore. This happens when you overheat the engine so much that the pistons expand and the bore expands together closing the gap on the ring causing huge increases in friction and an interference fit. The easiest way to fix this is to remove the spark plugs and squirt a little oil in, then with the plugs out, turn the crank backwards until it's breaks the rings off the cylinder bore. Then keep rotating it a little. Next remove dizzy, prime the oil system up good and try and start it up.
When a rod bearing is shot really bad things happen. The rod usually friction welds itself to the crank and the resulting mess is a damaged block, crank, rod, etc. When a cam bearing seizes things aren't nearly as bad but the engine will need to be taken all apart to get to the cam bearings which are pressed in. Then you've got the main crank bearings. Those seize and you'll need a new crank or at least have it checked out and polished, then the block should get align honed.
Also when a bearing seizes you need to find out why before you go replacing the bearings and throughing everything back together. There is usually a clog somewhere in the oil passages. All it takes is a good overheating and you can spin a bearing, that's why tolerances are SO important when blue printing.