Cameras can create two kinds of rendering effects: depth of field and motion blur.
Motion blur applied to wings of the flying dragon
Multi-pass rendering effects use multiple renderings of the same frame, with slight camera movement between each rendering. The multiple passes simulate the blurring that film in a camera would register under certain conditions.
Although it is not a multi-pass effect, the choices in the drop-down list also let you specify the depth-of-field value for the mental ray renderer. See Depth of Field Parameter (mental ray Renderer).
On the Parameters rollout, a “Depth Of Field (mental ray)” choice supports the mental ray renderer's depth-of-field effects. To use this, turn on Enable in the camera's Multi-Pass Effect group. Also turn on Depth Of Field on the Camera Effects rollout of the Render Setup dialog.
Cameras can generate depth-of-field effects. Depth of field is a multi-pass effect. You turn it on in the Parameters rollout for cameras. Depth of field simulates a camera's depth of field by blurring areas of the frame at a distance from the camera's focal point (that is, its target or target distance).
Cameras can generate motion blur effects. Motion blur is a multi-pass effect. You turn it on in the Parameters rollout for cameras. Motion blur simulates the motion blur of a camera by offsetting rendering passes based on movement in the scene.
Create panel 
(Cameras)