[vore]

Concurrency is not parallelism:

- Concurrency is making overlapping progress on two or more tasks.

- Parallelism is making simultaneous progress on two or more tasks.

Concurrent tasks can run in parallel, but they can also run not in parallel and still make overlapping progress by either cooperative or preemptive scheduling. You can imagine that in some larger tasks you have these 3 second sleeps, but all tasks are able to make overlapping progress without blocking the other ones -- that's concurrency!

[avinassh]

There is a great talk by Rob Pike: Concurrency is not Parallelism - https://www.youtube.com/watch?v=oV9rvDllKEg&t=23s