refers to hardware and software of quantum computers, and what their capabilities will be. For protecting your data against a quantum attacker, see [post-quantum].
Quantum computers are an upcoming technology that leverages the power of quantum physics to perform computations. They take advantage of quantum superposition in order to gain speedups ranging from quadratic (Grover's Algorithm) to exponential (Shor's Algorithm) in certain special problems.
Quantum computers are not general purpose computers: quantum computers are fundamentally incapable of doing most of the computations that we take for granted on classical computers. So while these speedups sound impressive, they only apply to a very specific set of problems.
However, two of the problems where quantum computers have an advantage include factoring numbers into their primes (exp. speedup), and brute-forcing a key (quadratic speedup), which makes quantum computers of interest to information security professionals.
The threat of quantum computing is not imminent: in 2014 professor Matteo Mariantoni gave a talk in which he estimated that the earliest possible date for a large-scale quantum computer is the year 2030 at a cost of 1 billion dollars for research, construction, and the required nuclear power plants to operate it.