Most DSLRs have three distinct autofocus modes, and it's important to understand the differences between them. In this article, we'll look at their different functions.
One Shot / Single Shot / AF-S
Single Shot is the autofocus mode that most DSLR photographers use with their cameras, and it is definitely the one to start with as you learn how to use your DSLR. It's best to practice in this mode while shooting static photos, such as landscapes or still life. In Single Shot mode, the camera needs to be re-focused every time you move the camera, and -- as the name suggests -- it will only shoot a single shot at a time.
AI Servo / Continuous / AF-C
The AI Servo (Canon) or AF-C (Nikon) mode is designed for use with moving subjects (such as wildlife photography or sports photography). The shutter button is half-pressed to activate focusing, as usual, but there won't be any beeps from the camera or lights in the viewfinder. In this Continuous mode, as long as the shutter is half-pressed, you can track your subject as it moves, and the camera will keep re-focusing.
AI Focus / AF-A
This mode combines both the above-mentioned autofocus modes. In AI Focus (Canon) or AF-A (Nikon), the camera remains in Single Shot mode unless the subject moves, in which case it automatically switches to Continuous mode. The camera will emit a soft beep once the subject is focused. This can be particularly useful for photographing children, who are inclined to move around quite a lot!