A Process A process or task is a portion of a program in some stage of execution. A Thread A thread is a separate part of a process. A process can consist of several threads, each of which execute separately. This gives good response times for complex programs. Windows NT is an example of an operating system which supports multi-threading.
Some systems run only a single process at a time, other systems run multiple processes at once.
The simple answer is that it doesn't. The processor of the computer runs one process for a short period of time, then is switched to the next process and so on. As the processor executes millions of instructions per second, this gives the appearance of many processes running at once. In a computer system which supports more than one process at once, some mechanism must be used to switch from one task to another. There are two main methods used to perform this switching.