A process is a collection of virtual memory space, code, data, and system resources. A thread is code that is to be serially executed within a process. A processor executes threads, not processes, so each application has at least one process, and a process always has at least one thread of execution, known as the primary thread. A process can have multiple threads in addition to the primary thread. Prior to the introduction of multiple threads of execution, applications were all designed to run on a single thread of execution.
When a thread begins to execute, it continues until it is killed or until it is interrupted by a thread with higher priority (by a user action or the kernel?s thread scheduler). Each thread can run separate sections of code, or multiple threads can execute the same section of code. Threads executing the same block of code maintain separate stacks. Each thread in a process shares that process?s global variables and resources.