A quantum bit or qubit, as opposed to a classical one, can be in the zero and one state at the same time. Find out how this is possible in this video.
A qubit can be in in a superposition of 0 and 1, simultaneously representing two values. Suppose we have 4 qubits: how many values can this system (simultaneously) represent? What if we had 1000 qubits?
Quantiki is an excellent open wiki on quantum information.