Difference between Concurrent Processing and Parallel Processing

Both ‘Concurrent Processing’ and ‘Parallel Processing’ refer to multiple processes executing on the CPU within a period,
but they are two different things.

According to The Art of Concurrency,
Concurrent means:

two or more processes are in progress at the same time

While Parallel means:

two or more processes executing simultaneously

They look pretty similar but are actually different.
For example,
two processes are executing,
process A and process B.

Parallel Processing

Parallel processing may look like this：

Both Process A and Process B are being executed.

Concurrent Processing

However, for concurrent processing,
the execution might look like the diagram above,
or it might look like this:

Both Process A and Process B are in progress,
but they are not executing at the same time.

Notice

Parallel processing is only a type of concurrent processing.
As long as there are multiple processes in progress,
it is concurrent processing.

There are many ways to achieve concurrent processing,
and parallel processing is only one of them.