Cameras can create two kinds of rendering effects: depth of field and motion blur.
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Create panel > 
(Cameras) > Target or Free > Parameters rollout > Multi-Pass Effect group
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, iray, and Quicksilver Renderers).