Pressing the shutter-release button halfway initiates autofocus. Focus mode lets you choose whether focus locks once the camera has focused or if the camera continues to adjust focus while the shutter-release button is pressed halfway.

AF-A (Auto-servo AF)
Camera automatically selects single-servo autofocus if subject is stationary, continuous-servo autofocus if subject is moving. Shutter can only be released if camera is able to focus.

AF-S (Single-servo AF)
For stationary subjects. Focus locks when shutter-release button is pressed halfway. Shutter can only be released if camera is able to focus.

AF-C (Continuous-servo AF)
For moving subjects. Camera focuses continuously while shutter-release button is pressed halfway. Shutter can only be released if camera is able to focus.

MF (Manual focus)
Focus manually.

AF-S

AF-S

AF-C