Concurrency is the ability of parts of a program to work correctly when executed out of order.
Concurrent execution, on the other hand, alternates doing a little of each task until both are all complete.
Concurrency allows a program to make progress even when certain parts are blocked.
When tasks don’t just interleave, but run at the same time, that’s called parallelism. Multiple CPU cores can run instructions simultaneously.
When a program – even without hardware parallelism – switches rapidly enough from one task to another, it can feel to the user that tasks are executing at the same time. You could say it provides the “illusion of parallelism.” However, true parallelism has the potential for greater processor throughput for problems that can be broken into independent subtasks. Some ways of dealing with concurrency, such as multi-threaded programming, can exploit hardware parallelism automatically when available.