On the barrel of AF-S NIKKOR lenses there is a focus mode switch for autofocus or manual focus modes. Each mode offers benefits as described below.
M/A focus mode
The M/A mode allows switching from autofocus to manual operation, by simply rotating the focus ring, with virtually no time lag. This is a popular option for sports photographers who autofocus on their subject, but who require fine focus with manual control and an instant response to the manual adjustment. This option is common on a wide range of lenses including professional lenses due to the fine control and the responsiveness it offers.
A/M focus mode
The A/M focus mode also offers autofocus with manual override the same as M/A, however the focus ring override is less sensitive than in M/A to prevent unintentionally switching to manual mode. So fine manual focus adjustment is possible in autofocus, but the speed of reaction is not as high as with M/A mode. A/M mode is more often found on mid-range AF-S NIKKOR lenses and occasionally as an additional mode on lenses with an M/A mode.
A-M mode switch
The A-M mode switch allows switching either to autofocus or manual focus. When in autofocus the focus ring cannot be used to manually focus the lens, you must switch to manual focus to do this. When in manual mode operation of the lens is loaded to achieve the same feel as when using a manual focus lens by providing some resistance. This switch appears most often on AF-S NIKKOR DX lenses.