Depth of Field in Animation

Depth of field is the region of focus of a camera.
The camera can focus only a particular distance from itself (which depends on the focus setting of the lens). The area ahead and beyond this focus zone gets blurred gradually. The range of this focus area can be longer or shorter depending on the setting (aperture) of the camera.

In 3D animation software such as Maya or Max, one can render a separate Z depth / L depth layer of the scene, which stores the distance information in greyscale (objects far from camera are rendered as black and closer in white or vice-versa). This z depth layer could be imported along with rendered images in compositing software such as Nuke or Fusion to produce the illusion of depth of field (A separate plugin may be required to tweak the level of blur and zone you want in focus).