Most DSLR cameras have three AUTO-FOCUS modes in which to function. Your choice of function will determine how the camera achieves focus on your subject.
*TIP* To focus on your subject without taking an image, press the shutter button half way down. There might also be a button option on the back of the camera you can designate (through your menu system) to serve the same function as holding the shutter button half way down.
The following list compares Canon and Nikon AF-Mode terminology.
CANON NIKON PURPOSE
One Shot Single Area Selects focal point once shutter is pressed half way down
AI Focus Continuous Camera predicts if subject is moving. If so, goes into full-time focus adjusting
AI Servo Full-time Camera continuously tracks moving subject