One of the misconceptions about concurrency is people often confuse concurrency and parallelism. Concurrency is a programming model that lets you express things that are independent, as independent executions and parallelism is about running two things at the same time. They are not the same idea and a lot of people confuse them, they think they are the same thing, but concurrency and parallelism are best thought of as a different idea. You can write beautiful concurrent programs with no parallelism whatsoever and you can also write extremely parallel programs that are not remotely concurrent. Concurrency is a model, parallelism is a result – maybe that’s the best way to say it.

- Rob Pike

Source: Infoq