What is the difference between concurrency and parallelism?

Concurrency means multiple operations can make progress during overlapping periods of time. They do not necessarily execute at exactly the same instant.

Parallelism means multiple operations actually execute simultaneously, usually on multiple CPU cores.

Async/await is commonly used for concurrency with I/O-bound operations such as database and HTTP calls. Parallel APIs are more relevant to CPU-bound work that can safely be divided across cores.

Using parallelism in ASP.NET Core request handling should be deliberate because excessive CPU work or unnecessary parallel tasks can reduce overall server throughput.