Processes are managed by the scheduler of the OS. The scheduler can either let a process run until it ends or blocks (non-interrupting scheduler), or it can ensure that the currently running process is interrupted after a short period of time. The scheduler can switch back and forth between different active processes (interrupting scheduler), alternatively assigning them CPU time. The latter is the typical scheduling strategy of any modern operating system. This is pretty resource intensive, so the OS supports a more resource-friendly way of handling concurrent operations: threads.
