Laws of quantum physics allow bits to be in multiple states simultaneously so it has the potential to be millions of times more powerful than today's most powerful supercomputers.
Quantum bits, or Qubits are more versatile than standard bits because they can exist in three states instead of two. Current computers represent things as a one or zero, but a quantum computer can render a qubit as representing a one, a zero, or every fraction between one and zero at the same time.
An interesting thing about qubits is that by just looking at one, it changes its state, so scientists had to devise a way to look without the qubit knowing it was being looked at. (Long story, but fascinating)
A 30-qubit quantum computer is approximately as powerful as a 10 teraflop computer. It can solve 10 trillion floating point operations every second vs. an average computer, which performs about seven gigaflops (seven billion) per second. Quantum computers process multiple calculations at once vs. current computers, which process one at a time.
Google and NASA have a 512-qubit quantum computer housed in a 10 foot black cabinet, but do not expect to buy one for your home in the near future. The NASA Ames machine may be upgraded to a 2,048 qubit chip in the next year or two. There are 25.4 million nanometers in one inch and fingernails grow one nanometer every second.