When photographers talk about the “depth of field” in an image, they’re referring to how much of the picture they choose to have in focus. You may think “but don’t I want all of my image to be in focus?” Frequently you do, but sometimes you can bring more attention to your subject by reducing the depth of field in a scene, making the background soft and blurry.

This image has very deep depth of field. Everything from the foreground to the horizon is in focus. You'll usually choose deep depth of field for landscape shots.
If you own a DSLR but aren’t experimenting with your aperture settings, you’re missing out on one of the most useful tools in a photographer’s arsenal. Here’s a primer to get you started.
How to use depth of field to take better pictures [Macworld]