PPT Slide
Process - has its own address space and is allocated CPU time.
Process with threads - split out the concept of CPU allocation into the thread, leaving the process as a shell to hold everything else.
Threads can be implemented in the operating system or they can be implemented totally within a user process (called user threads).
A user process does not need to be in system mode and use special instructions to swap stack pointers and register sets. A timer helps, though.
User threads are more efficient (no O/S switch).