What concurrency really means
In a small computer, with a single processor and a single core, all evaluations are serialized only through the core of the processor. The operating system will interleave multiple processes and multiple threads through clever time-slicing arrangements.
On a computer with multiple CPUs or multiple cores in a single CPU, there can be some actual concurrent processing of CPU instructions. All other concurrency is simulated through time slicing at the OS level. A Mac OS X laptop can have 200 concurrent processes that share the CPU; this is far more processes than the number of available cores. From this, we can see that the OS time slicing is responsible for most of the apparently concurrent behavior.
