Detect when apps are started

Hi Guys,
There are a few 'app lock' apps out there that request a password before they give access to an app you select.

I'm working on a different project but i'd like to detect when the user starts other apps. I know it can be done because somehow these 'app lock' apps fire each time I open, for example, my messaging app that I chose to protect within the app lock app

Hope that makes sense. Anyone know which classes I should be looking at to accomplish this?
Cheers
Ian