A thread is a path of execution through a program. It is an executable entity that belongs to one and only one process. Each process has at least one thread of execution, automatically created when the process is created. Your main program runs in the first thread. A Win32 thread consists of a stack, the state of the CPU registers, a security context, and an entry in the execution list of the system scheduler. Each thread shares all of the process's resources.
A process consists of one or more threads and the code, data, and other resources of a program in memory. Typical program resources are open files, semaphores (a method of interthread communication), and dynamically allocated memory. A program executes when the system scheduler gives one of its threads execution control. The scheduler determines which threads should run and when they should run. Threads of lower priority might need to wait while higher priority threads complete their tasks. On multiprocessor machines, the scheduler can move individual threads to different processors to balance the CPU load.
Because threads require less system overhead and are easier to create than an entire process, they are useful for time- or resource-intensive operations that can be performed concurrently with other tasks. Threads can be used for operations such as background printing, monitoring a device for input, or backing up data while it is being edited.
When threads, processes, files, and communications devices are opened, the function that creates them returns a handle. Each handle has an associated Access Control List (ACL) that is used to check the security credentials of the process. Processes and threads can inherit a handle or give one away using functions described in this section. Objects and handles regulate access to system resources. For more information on handles and security, see the Win32 Application Programming Interface reference (such as the Platform SDK online title).
All threads in a process execute independently of one another. Unless you take special steps to make them communicate with each other, each thread operates while completely unaware of the existence of other threads in a process. Threads sharing common resources must coordinate their work by using semaphores or another method of interthread communication. For more information on interthread communication, see Sharing Resources.