"The premise behind a quantum computer is simple - provided you swallow the unpalatable quantum truths that underlie it. One is that objects such as atoms and electrons are not confined to being either this or that, as the objects of our everyday macroscopic world are; they can be both this and that at the same time. They might, for instance, be spinning clockwise and anticlockwise simultaneously, or adopt two different energy states at once. This is known as superposition.
What's more, these ambiguous quantum characters can club together so that what you do to one affects the others. This is the phenomenon of entanglement or, if you're Einstein, "spooky action at a distance". Together, the characteristics of superposition and entanglement make for a computer of awesome power.
Take a classical computational bit such as a transistor current. It can adopt one of two states: 0 (off) or 1 (on). Not so its quantum counterpart, the qubit. Superposition means a single qubit can simultaneously be 0 and 1, giving you twice the information storage capacity right from the start. Then entanglement kicks in, allowing further bits to share their superposed states in a common pool. The result is that computing power grows exponentially with the number of qubits. While three classical bits are needed to store the number 7, three qubits can store all eight numbers from 0 to 7 simultaneously
Michael Brooks, New Scientist, 21 September 2009
Blimey! I don't really understand this stuff, but I love the hyperbole that turns out to be simple fact.