Basically: Qubits (sort of) encode all possible states that a standard string of bits can take at the same time as a superposition, such that when you measure them, you have a possibility of observing each possible setting of the bits.

You can manipulate the possibility of observing certain states by performing operations on the bits (which are effectively interference). So a quantum calculation is more a probabilistic restriction on which state you want, rather than a direct calculation. In order to be sure of a result, you need to repeat the calculation to get a desired confidence (or just check the answer directly if that would be faster).