[T-R]

Concurrency = managing shared resources. Things like mutual exclusion and transactions. Getting processors to get in line (serialization/composition) for a shared resource instead of just trampling over each other by just grabbing it when they want (non-determinism), so that actions that depend on being done in order and without interruption can be done.

Parallelism = splitting up subtasks that don't depend on each other. If the tasks don't depend on each other, the things doing them shouldn't need to talk. If you have enough processors, you can do almost everything at once, so the algorithm is only as slow as things that actually need to be done in order (asymptotic efficiency).

In imperative programming, we tell the computer what order to do things in, so it doesn't know what actions don't depend on each other - to parallelize, we have to worry about concurrency. In functional programming, the compiler knows what depends on what (what uses the result of something else), so it can parallelize things that don't depend on each other. It might be implemented with concurrency, but the programmer doesn't need to worry about it.