Parallel versus concurrent processing

When we hear about working threads executing at the same time, we automatically assume that they literally do what they are programmed to do in parallel. Only after we look under the hood of such a system do we realize that such parallel processing is possible only when the threads are each executed by a different CPU; otherwise, they time-share the same processing power. We perceive them working at the same time only because the time slots they use are very short—a fraction of the time units we use in our everyday life. When threads share the same resource, in computer science, we say they do it concurrently.