A succinct way to distinguish both is to focus on what problem they solve:
> Concurrency is concerned about correctness, parallelism concerned about performance.
Concurrency is concerned about keeping things correct[1] when multiple things are happening at once and sharing resources. The reason why those problems arise might be for performance reasons, e.g. multiplexing IO over different threads. As such, performance is still a concern. But, your solution space still involves the thread and IO resources, and how they interleave.
Parallelism is in a different solution space: you are looking at the work space (e.g. iteration space) of the problem domain and designing your algorithm to be logically sub-dividable to get the maximum parallel speedup (T_1 / T_inf).
Now, a runtime or scheduler will have to do the dirty work of mapping the logical subdivisions to hardware execution units, and that scheduler program is of course full of concurrency concerns.
[1] For the sake of pedantry: yes, parallelism is sometimes also used to deal with correctness concerns: e.g. do the calculation on three systems and see if the results agree.
> Concurrency is concerned about correctness, parallelism concerned about performance.
Concurrency is concerned about keeping things correct[1] when multiple things are happening at once and sharing resources. The reason why those problems arise might be for performance reasons, e.g. multiplexing IO over different threads. As such, performance is still a concern. But, your solution space still involves the thread and IO resources, and how they interleave.
Parallelism is in a different solution space: you are looking at the work space (e.g. iteration space) of the problem domain and designing your algorithm to be logically sub-dividable to get the maximum parallel speedup (T_1 / T_inf). Now, a runtime or scheduler will have to do the dirty work of mapping the logical subdivisions to hardware execution units, and that scheduler program is of course full of concurrency concerns.
[1] For the sake of pedantry: yes, parallelism is sometimes also used to deal with correctness concerns: e.g. do the calculation on three systems and see if the results agree.