December  20 2019: Google's  CEO Sundar Pichai  explains the  key difference between classical and quantum computing in a recent blog:
A bit in a classical computer can store information as a 0 or 1. A quantum bit—or qubit—can be both 0 and 1 at the same time, a property called superposition. So if you have two quantum bits, there are four possible states that you can put in superposition, and those grow exponentially. With 333 qubits there are 2^333, or 1.7x10^100—a Googol—computational states you can put in superposition, allowing a quantum computer to simultaneously explore a rich space of many possible solutions to a problem.
Quantum computers are prone to errors, yet our experiment showed the ability to perform a computation with few enough errors at a large enough scale to outperform a classical computer. As we scale up the computational possibilities, we unlock new computations. We are able to achieve these enormous speeds only because of the quality of control we have over the qubits.
For those of us working in science and technology, it's...the most meaningful milestone to date in the quest to make quantum computing a reality.
Read  our lead story on Quantum Computing here