Depth of field photography definition
Depth of field is the total area, from front to back, that is in focus (or sharp – sometimes called “acceptably sharp”) in an image.
Think of it like this, a lens only focuses on one particular distance in the shot (the subject of the photo) and therefore this part of the image will be sharp. Depth of field is gradual and anything in front or behind the subject will slowly begin to become more blurry the further away it is from the focal point of the picture. This of course varies depending on the camera and also on the settings, such as aperture.
Put simply, a large depth of field means most of your photo will be in focus. Think landscapes. A shallow depth of field will keep one part of your photo in focus, that being the subject. Think a person standing in front of that landscape.
How to control depth of field
Aperture(f-stop) can be used to control depth of field. A large aperture (small number – I know, confusing) will decrease depth of field.
A small aperture (large number) will increase depth of field.
Focal lens length - Lens strength limits the aperture capability. A higher magnification will result in a shallower depth of field, e.g. 70mm will produce a larger depth of field than a 300mm.
One other factor affecting the depth of field in your photography is the distance of the subject.
The video below is just over 15 minutes but is a really good tutorial on depth of field.