Comment by titzer

Parallelism and concurrency are different models. It is not a needless distinction.

With just concurrency, you can have a system, e.g. an event processing system, where each event happens, is processed to completion, and then the next event is processed. Events can be of different types and can have different handlers. Events can be arrive (or be delivered) in different orders. The context-switching points are known (in this case, the beginning and end of event processing). As such, any computation between context switching points appears to happen atomically (i.e. no partial update, no internal reordering).

Parallelism is a different model.

There are no well-defined, limited set of interaction points between computations. A computation could literally be interrupted at any machine-level instruction and another computation start running. Suddenly, the number of possible interactions is huge--almost beyond comprehension. Locks, transactions, or other synchronization mechanisms are necessary in order to create larger atomic regions (and defend against race conditions). Worse, with weak memory models, which most modern multi-core hardware have, means that interleaving alone is not enough to explain the possible interactions between threads. It's possible with weak memory models that writes from another computation appear in different orders to different threads. Weak memory models make avoiding race conditions absolutely paramount.