AF system can work in differnent mode:
Single AF- Basic AF mode. In this mode camera autofocus the object in AF point(s) in the moment of half pressing shutter.
AF servo- When we want to take a photo of moving object single AF mode is not best mode for us. With AF servo we can track the object to make it always in focus and this alow to us to make a sharp photos of moving object.
Autofocusing detect the different in contrast in the scene. So when is scene light and contrast the AF works fast and nice and in low light situations AF can work poor or unusable complitly. To reduce this some companies put a AF assist lamp with normal or infrared light. This can help to us a lot but this is not a perfect sollution for every situation.
I want notice one thing. AF system work better on SLR cameras(digital or analog) than on compact cameras. There are lot reasons for this but this will be explained in some other chapter.
View thru viewfinder on Canon EOS 20D