Concurrency is when two tasks can start, run, and complete in overlapping time periods. Parallelism is when tasks literally run at the same time, eg. on a multi-core processor.
Concurrency is the composition of independently executing processes, while parallelism is the simultaneous execution of (possibly related) computations.
Concurrency is about dealing with lots of things at once. Parallelism is about doing lots of things at once.
An application can be concurrent – but not parallel, which means that it processes more than one task at the same time, but no two tasks are executing at same time instant.
An application can be parallel – but not concurrent, which means that it processes multiple sub-tasks of a task in multi-core CPU at same time.
An application can be neither parallel – nor concurrent, which means that it processes all tasks one at a time, sequentially.
An application can be both parallel – and concurrent, which means that it processes multiple tasks concurrently in multi-core CPU at same time.
Vipin Jain. Differences between concurrency vs. parallelism