Simply put, depth of field is how much of the background is in focus.

If the background is blurry and the subject is in focus, that’s referred to as shallow depth of field. For example, setting your aperture to the lowest number, like f/2.8. Here’s an example of shallow depth of field. I had my 50mm lens aperture set to the lowest number: f/1.6 (confusingly referred to as a “large” aperture setting, because the lens is wide open, letting in the maximum amount of light):

If the background is in focus, along with the foreground, that’s maximum depth of field. For example, setting your aperture to f/22. Below is an example of a photo with maximum depth of field (I think this was f/29). See how both the foreground and background are in focus?

Shallow depth of field is a useful technique used in portraits, or when you want to isolate just one portion of the photo and draw attention to it. See my post on How to Make the Background Fuzzy for a tutorial on using depth of field for portraits.

Maximum depth of field is great for landscape photos, where you want to show the vastness of the scene. Check out the post on how to include a strong foreground element in your landscape photos for a couple of examples that illustrate the strength of using maximum depth of field.