Quick explanation. Flipping a bit in classical computing is an irreversible operation, and so has to release heat. We also have to be in one state at a time.

Quick explanation. Flipping a bit in classical computing is an irreversible operation, and so has to release heat. We also have to be in one state at a time. In quantum computing, we are only allowed reversible operations, but can be in a superposition of states. There is no heat released, no upper limit on how fast the operations can go, and no upper limit on parallelism. But the flip side is that you cannot peek at the operation, and error correction is not exactly easy. The "only reversible operations" bit is a huge restriction. For example it means that "if" statements are not allowed.