Know other app start and close

I just want to know how to know an app is started and closed. I just want to find a way that when skype is started, a service on the background will dete开发者_StackOverflow中文版ct it and then automatically start another app to collect the sensor data. Dose anybody have some ideas about that?

It seems like a job for a Broadcast Receiver. There's a reasonable explanation here - I've never used them, but I was wondering how to do the same thing a week or so back, and half-reading that page is about as far as I got. If you get it working, please post back here with your results.