What is the difference between concurrency and parallelism?

Question

What is the difference between concurrency and parallelism?

Examples are appreciated.

Answer 1

I really liked this graphical representation from another answer - I think it answers the question much better than a lot of the above answers

Parallelism vs Concurrency When two threads are running in parallel, they are both running at the same time. For example, if we have two threads, A and B, then their parallel execution would look like this:

CPU 1: A ------------------------->

CPU 2: B ------------------------->

When two threads are running concurrently, their execution overlaps. Overlapping can happen in one of two ways: either the threads are executing at the same time (i.e. in parallel, as above), or their executions are being interleaved on the processor, like so:

CPU 1: A -----------> B ----------> A -----------> B ---------->

So, for our purposes, parallelism can be thought of as a special case of concurrency

Source: Another answer here

Hope that helps.