a thread of execution is the smallest sequence of programmed instructions that can be managed independently by an operating system scheduler. For more detailed jump to : http://en.wikipedia.org/wiki/Thread_(computing)

the above code runs 4 threads in a single process, where each thread utilising resources of a common process initiated. So this is probably doing it with single processor when my macbook has 1 processor and 2 cores.

However keeping in mind with the python code above,  each subsequent threads will starts in each loop and do their operation. Python suffers from GIL, so you should consider using threads only in case of i/o bound tasks and not computational one.

Multiple threads can exist within the same process and share resources such as memory, while different processes do not share these resources. In particular, the threads of a process share the latter's instructions (its code) and its context (the values that its variables reference at any given moment).

Here is a simple program that downloads mp3 files from internet parallel however the display is not done in the same way due to the nature how sys.stdout.write & sys.stdout.flush() works

Later that day, I used Queue & daemon thread, see here

threading can be complicated when threads need to share data or resources. The threading module does provide many synchronization primatives, including semaphores, condition variables, events, and locks. While these options exist, it is considered a best practice to instead concentrate on using queues. Queues are much easier to deal with, and make threaded programming considerably safer, as they effectively funnel all access to a resource to a single thread, and allow a cleaner and more readable design pattern.

“Join”  method is used for waiting on threads & processes to end.  It means “wait until the thread dies”, without explicitly killing it. So "join" doesn’t kill the thread, it just waits (and blocks) until the thread has finished running. 

In non-daemon thread, you'd have to keep track of them, and tell them to exit, before your program can completely quit. By setting them as daemon threads, you can let them run and forget about them, and when your program quits, any daemon threads are killed automatically.

A daemon thread is ended when it's parent thread has ended, a non-daemon thread will keep the parent thread alive until the non-daemon is finished.

In the coming days I will go overhyper-threading with examples