What is the difference between concurrency and parallelism?

Question

What is the difference between concurrency and parallelism?

Examples are appreciated.

Answer 1

I'm going to offer an answer that conflicts a bit with some of the popular answers here. In my opinion, concurrency is a general term that includes parallelism. Concurrency applies to any situation where distinct tasks or units of work overlap in time. Parallelism applies more specifically to situations where distinct units of work are evaluated/executed at the same physical time. The raison d'etre of parallelism is speeding up software that can benefit from multiple physical compute resources. The other major concept that fits under concurrency is interactivity. Interactivity applies when the overlapping of tasks is observable from the outside world. The raison d'etre of interactivity is making software that is responsive to real-world entities like users, network peers, hardware peripherals, etc.

Parallelism and interactivity are almost entirely independent dimension of concurrency. For a particular project developers might care about either, both or neither. They tend to get conflated, not least because the abomination that is threads gives a reasonably convenient primitive to do both.

A little more detail about parallelism:

Parallelism exists at very small scales (e.g. instruction-level parallelism in processors), medium scales (e.g. multicore processors) and large scales (e.g. high-performance computing clusters). Pressure on software developers to expose more thread-level parallelism has increased in recent years, because of the growth of multicore processors. Parallelism is intimately connected to the notion of dependence. Dependences limit the extent to which parallelism can be achieved; two tasks cannot be executed in parallel if one depends on the other (Ignoring speculation).

There are lots of patterns and frameworks that programmers use to express parallelism: pipelines, task pools, aggregate operations on data structures ("parallel arrays").

A little more detail about interactivity:

The most basic and common way to do interactivity is with events (i.e. an event loop and handlers/callbacks). For simple tasks events are great. Trying to do more complex tasks with events gets into stack ripping (a.k.a. callback hell; a.k.a. control inversion). When you get fed up with events you can try more exotic things like generators, coroutines (a.k.a. Async/Await), or cooperative threads.

For the love of reliable software, please don't use threads if what you're going for is interactivity.

Curmudgeonliness

I dislike Rob Pike's "concurrency is not parallelism; it's better" slogan. Concurrency is neither better nor worse than parallelism. Concurrency includes interactivity which cannot be compared in a better/worse sort of way with parallelism. It's like saying "control flow is better than data".