Concurrency means structuring a program so many tasks can make progress in overlapping time windows. Parallelism means literally running tasks at the same instant on separate cores. A single-core machine can be concurrent but never truly parallel.
A concurrent design interleaves tasks by pausing one and resuming another. This helps when tasks wait on input, network, or disk. Parallelism helps when you have real compute to split across cores.
You can have either without the other. A single-threaded event loop is concurrent, not parallel. A numeric loop spread across eight cores is parallel. Most real systems mix both.
The practical call: reach for concurrency when tasks mostly wait, and parallelism when tasks mostly compute. Confusing them leads to adding cores that never help, or adding threads to work that has no waiting to overlap.
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