As a programmer, you probably have already some understanding about what a process is. As operating systems look at it, a process is a rough equivalent of an application. When a user starts an application, an operating system creates and starts a new process. The process owns the application code and all the resources that this code uses—memory, file handles, device handles, sockets, windows, and so on.

When the program is executing, the system must also keep track of the current execution address, state of the CPU registers, and state of the program's stack. This information, however, is not part of the process, but of a thread belonging to this process. Even the simplest program uses one thread. 

In other words, the process represents the program's static data while the thread represents the dynamic part. During the program's lifetime, the thread describes its line of execution. If we know the state of the thread at every moment, we can fully reconstruct the execution...