Concurrency is when two or more tasks can start, run, and complete in

overlapping time periods. It doesn't necessarily mean they will ever both be

running at the same instant. Eg. multitasking on a single-core machine.

Parallelism is when tasks literally run at the same time, eg. on a multicore

processor.

Quoting Sun's Multithreaded Programming Guide:

- Concurrency: A condition that exists when at least two threads are

makingprogress. A more generalized form of parallelism that can include

timeslicing as a form of virtual parallelism.

- Parallelism: A condition that arises when at least two threads

areexecuting simultaneously