Processes, Threads, and Scheduling

A program on disk is passive. A process is the operating system's running instance of that program. It holds resources such as an address space, open files, credentials, and one or more threads. A thread is an execution path within that process. It has execution state and can run instructions, block for work, or become runnable again.

This distinction gives you a useful starting point for concurrent systems. A process is usually a protection and ownership boundary. Threads inside it can coordinate through the same process resources, but that convenience creates shared-state risks. Separate processes need an explicit communication mechanism, but a failure in one process is more readily contained by its resource boundary.

What the scheduler does