What is the difference between concurrency and parallelism?

Question

What is the difference between concurrency and parallelism?

Examples are appreciated.

Answer 1

Say you have a program that has two threads. The program can run in two ways:

Concurrency                 Concurrency + parallelism
(Single-Core CPU)           (Multi-Core CPU)
 ___                         ___ ___
|th1|                       |th1|th2|
|   |                       |   |___|
|___|___                    |   |___
    |th2|                   |___|th2|
 ___|___|                    ___|___|
|th1|                       |th1|
|___|___                    |   |___
    |th2|                   |   |th2|

In both cases we have concurrency from the mere fact that we have more than one thread running.

If we ran this program on a computer with a single CPU core, the OS would be switching between the two threads, allowing one thread to run at a time.

If we ran this program on a computer with a multi-core CPU then we would be able to run the two threads in parallel - side by side at the exact same time.