Threads are processes that contain a program counter, a register set and a stack space. They simply share their ode section, data section, and operating systems resources with other processes.
The CPU switches from processes so much that the use of threads to switch among peer threads makes it easy for the CPU because it does not have to do context switches which menas no memory-management work. The advantage is that when a system implements user-level threads in user-level libraries, thread switching is not needed and can be done independently of the operating and therefore, very quickly.
