In my understanding: The autofocus system relies on the sensor information you give it and puts its through some form of algorithm to determine if the subject is in focus.
You're close, at least when talking about point & shoot and mirrorless cameras. They use an AF system called "contrast detect", which means that for a given focus field, the camera racks the focus back and forth and studies the area for the greatest contrast between a border, the greatest contrast being the point where focus is achieved. In contrast detect focus, an AF point can be anywhere in the frame, which is why for things like some of the Lumix G mirrorless cameras you can touch anywhere on the screen and have that be your focus point. The T4i will also have that feature when shooting in live mode, since live mode depends on contrast detect AF.
Traditional SLRs though use something called "phase detect" AF. These AF sensors bypass the camera's main sensor completely, and they work by measuring how the light comes through the lens, and whether the light is converging on the correct plane to be in focus. (This is really simplified, see here for more detail.) Phase detect is much, much faster than contrast detect.
I've often wondered maybe there are some tips as to what information is best to give the AF point i.e. are we best to choose a particular colour to focus on? What about picking something with a line on it, or a pattern?
Obviously this isn't ideal in all situations but would be good to know if something like this helps.
All AF systems depend on contrast to some degree to attain proper focus. Autofocus systems are as useless trying to resolve a featureless blank wall as they are trying to focus in pitch dark. If you can point your focus toward a strong border between light and dark, you'll be able to focus quickly.