i hope you won't regret asking this one:
The techinology that most SLRs use to focus is called Phase Detection, or Phase Difference Detection.
It works by first diverting of some of the image to the AF system. The AF system then divides the light into pairs (or phases) that are compared with each other for differences. The system is passive bcoz it has no moving parts and does no emit any light towards the subject.
Where i got the info from:
http://photonotes.org/other/ai-servo.html
Actually would appreciate if anyone has a link that explains phase-detection (wrt to AF) in more detail. Quite hard to find.