Definition: Depth of field refers to the focus of objects in a scene at various depths - such as the foreground, middle ground, and background. Blurring the focus of objects in the foreground seems to highlight objects in the background, while in reverse blurring the focus of objects in the background seems to highlight objects in the foreground. It's commonly used in live action filming, and can be easily accomplished by adjusting a camera's lens.
In animation it can be a bit more difficult; in 2d cel animation it can be accomplished by layering cells upon which one layer has blurred artwork, or by retouching 2d animation with computer software. In 3d animation it's a bit easier, generally accomplished by adjusting virtual cameras.
