Parallel computing is a type of software engineering where you use parallelism (e.g. more threads, more processes, more nodes, etc.) to increase the amount of computation power you use to tackle a problem. There are essentially three spheres of parallel computing: 1. CPU parallelism (OpenMP), 2. distributed parallelism (MPI), and 3. GPU parallelism (CUDA).

Parallel computing is related to performance engineering. This makes sense, because throwing more, but this is Amdahl's law. $$\text{Speedup}(N) = \frac{1}{(1 - P) + \frac{P}{N}}$$