What is Depth of Field? Simply put, Depth of Field is the amount of an image that is in focus.
If you shoot an outdoor scene that encompasses a one mile area with mountains in the background and a beautiful lake with reflections of the sky in it in the foreground of your image, you probably want all of that to be in focus. This would be a large depth of field and requires an f stop of 11 or more depending on the lens you are using.
If you are shooting a portrait of someone on a busy street in a downtown area, you will most likely want the subject to be very sharp with the buildings and cars on the street less sharp so as not to distract from the subject. This is shallow depth of field and requires an f stop of 5.6 or less depending on the lens you are using. If your subject is standing in front of a beautiful landscape you may want to use a greater depth of field so there is no exact rule for determining the DOF to use in a photo.
The preceding examples are extremes in either direction but offer good starting points to begin grasping the idea of DOF (depth of field). DOF is a very important aspect of photography. DOF is a judgement call and an expression of your vision as an artist. Learning to control it first and then shooting photos with an awareness of it will make you a better photographer.
If you want to practice and get a better idea of depth of field, take a series of shots of the same scene, going from the largest aperture your lens has (remember that smaller numbers are bigger apertures) to the smallest aperture (larger number).
If you are inside set something in the middle of the room in front of a wall that has pictures on it and shoot your subject with a series of shots progressing through the apertures on your lens. Your subject should be a few feet in front of the wall for the best results as opposed to being close to the wall. You should see that at the larger (smaller numbers) aperture settings of your images the pictures on the wall are out of focus and as the aperture decreases (larger numbers) the pictures come into focus.
If you are shooting images of wildlife such as birds, you really want to use a shallow depth of field. If all of the branches in trees are in focus if will make it difficult for people looking at your images to see the birds clearly. The same holds true for larger animals like deer. You don’t need the entire forest behind the deer to be in sharp focus for the viewer to know where the shot was taken. Use just enough DOF to get your subject in sharp focus.
For landscapes you generally want a large DOF, f/11 or more but for images of individual subjects less DOF generally works best. The image below may help you visualize DOF a bit better. Everything in red would be in focus at the given f stop. This is not an exact scale, but is should give you a good general idea.