Focusing Modes

Red-Shouldered Hawk (AI Focus AF)
One Shot would work here until he decides to fly

SLR cameras come equipped with four focusing modes, and all of them are very useful in different situations:

One Shot (Canon)/AF-S (Nikon).  This is the standard focusing mode photographers use most of the time.  When you press the shutter half-way, the camera focuses one time and maintains that focus until you take a picture or take your finger off the shutter button.  This mode is perfect for stationary subjects or subjects that are moving but not closer or farther away from you.

AI Servo (Canon)/AF-C (Nikon).  With this focusing mode, when you press the shutter half way, the camera focuses continuously on the subject.  It detects movement  in areas covered by the focus points (either all of them or the ones you've selected) and tries to maintain focus of that subject as it moves.  This is great for action and sports photography, especially when subjects are moving either toward you or away from you.  It helps you maintain focus on your subject.  The downside to this is that it will slightly lower the frame rate of you camera.  You will be able to take fewer pictures per second.

Red-Shouldered Hawk in Flight

AI Focus (Canon)/AF-A (Nikon).  This is a hybrid mode of the first two.  In this mode, the camera starts in One Shot/AF-S mode and will automatically switch to AI Focus/AF-A mode if it detects movement.  This is ideal for stationary subjects that may move at any second, like birds.  If you are shooting a Red-Shouldered Hawk perched on a branch, and he begins to fly, you don't want to take the time to switch autofocus modes.

Red-Shouldered Hawk Taking Flight
This is why I use AI-Focus AF for bird photography

Manual Focus.  Believe it or not, sometimes manual focus works better than autofocus.  When you're focusing on an area of the frame where there is no focusing point, and you don't want to recompose, manual focus works well.  In macro photography, I almost exclusively use manual focus.  If you have put a tele-extender on a lens to make your maximum aperture smaller than f/5.6, you will probably have to focus manually.  If you're concerned about having a sharp enough eye to manual focus correctly,  here's a trick to use.  Turn on live view mode and zoom in on the the area you want in focus, then set your focus manually.  I do this all the time when I put a 2x extender on a 400mm lens.

Immature Least Bittern, Manual Focus with 2x400mm lens

With the above photograph, the bird was so small and so far away, I needed a 2x tele-extender on my 400mm lens, which required me to manual focus.  So I put my camera in live view mode so that I could see the bird in the preview screen.  Then I zoomed in on the bird's eye and set my manual focus its eye.  Then I fired off a hundred shots, checking my focus every ten or so.  I don't know when I'll have the opportunity to see this again

Notes: Especially when manual focusing, make sure that your viewfinder's diopter is set properly for your eye.  There's a diopter adjustment you can use to make sure that what you see through the viewfinder is sharp.  Look for a dial next to your viewfinder.