Computer uses the sets of 0 and 1 for calculation. And use the nature of electricity for this function.
Quantum has not only the nature like 0 and 1 but also overlay and tunnel effect.
Quantum Computer uses the nature of quantum to improve speed of the calculation.
General computer serchs the best combination of 0 and 1 from specific combination of 0 and 1.
Quantum Computer starts same serch from ambiguous state.
General computer cannnot understand the difference local minimum and minimum when one of the local mimimum is serched.
Quantum computer can serch mimimum.
General computer uses the concept of probability with the combination of 0 and 1. So ambiguity of probability is made complexity and large amount of the combination.
Quantum computer uses quantum. And quantum itself has the nature of probability.