How to know the pid of active (focused) window?

I want to write a script in which it is necessary to know whether the user is actively using a program [browsing internet with somthing say firefox] or doing something else [writing text with something say gedit]

In my case i want to download big files but don't want to hurt browsering speed. so when i browse the active window is of firefox and i want to stop downloading. When i read pdf active window is of pdf reader and i want to resume download. That's it.