That threw me for a moment. In a forum, with threads, and asking how to kill them?
I guess you mean threads as in programs.
There are lots of ways of making a computer /appear/ to do more than one thing at a time (and some ways to literally have them do so).
One of the simplest mechanisms used within a single piece of code is "threading".
For an example of a very tiny bit of threading, I have a program on my website that does the "Countdown" numbers game. It uses a thread to do the solution, mainly because it originally was sooo slow. The UI let me abort a solution before it had finished. Clicking on the "Quit" button caused the main loop code to kill the thread it had started that was doing all the calculations.