3. multi-thread: different between process & thread

Processes
A process generally has a complete, private set of basic run-time resources; in particular, each process has its own memory space.
(To facilitate communication between processes, most operating systems support Inter Process Communication (IPC:进程间通讯) resources, such as pipes and sockets. IPC is used not just for communication between processes on the same system, but processes on different systems.)

Threads
Threads are sometimes called lightweight processes. Threads exist within a process — every process has at least one. Threads share the process's resources, including memory and open files.