You can have parallelism without much concurrency. Think parsing a bunch of files, where you have a `fn parse(path) -> AST` which does not rely on global state. Parallelizing something like this is trivial, with no mutexes in sight, and can be great for performance in many situations.

On the other hand, you can have concurrency without parallelism. Think a database where IO is the bottleneck, and you have multiple clients doing reading and writing at all once, potentially to the same table, in isolated transactions, on different db nodes which have to communicate. That's a lot of concurrency and nasty locks, even if you're running on a single core and wouldn't get much of a speedup from doing otherwise.