Depth Of Field depends upon a combination of aperture / f-stop (f8, f11, f16, etc) and the focal length of the lens used (e.g. 35mm lens, 85mm lens, 70-300mm zoom lens, etc).
Two things to remember:
1. The smaller the aperture i.e the greater the f-stop number (eg f16, f22, f32) the greater the depth of field.
2. The smaller the focal length of lens (eg. 50mm, 35mm, 28mm, 16mm) the greater the depth of field. The same applies to the focal length within a zoom lens. eg. for a 70-200mm zoom lens, the depth of field is greater at 70mm than at 200mm.
So a short lens like 28mm using an aperture of f16 will have a much larger depth of field than a 200mm zoomed lens at f16. And a 28mm lens with a large aperture of f1.8 will have a smaller depth of field than the same lens at f11.
The size of the depth of field will vary enormously depending upon the aperture/lens combination. For a short focal length and small aperture the depth of field may be measured in metres or even hundreds of metres. For a long telephoto and a large aperture, it may be cm. For a macro lens the Depth of Field may be measured in mm. Once you understand this and apply it, DOF becomes a great tool for planning your shot.
On many DSLRs there is a Depth Of Field preview button (usually beside the handgrip / lens). When you prees this and look through the viewfinder you can see what is in focus. However, at small apertures the image is very dark and difficult to see.
DOF & HYPERFOCAL DISTANCE
The Depth of Field also depends upon one very important thing - the Point of Focus. This is not always the same as the subject, as we shall see.
All lenses have an important characteristic. The depth of field in front of the Point of Focus (i.e. between the camera and the Point of Focus) is about 1/3 of the depth of field behind the Point of Focus.
This critical point of focus is called the HYPERFOCAL DISTANCE. It is the distance that places the furthest edge of the Depth of Field at infinity. The diagram below (from BigInMicroStock.com) explains this better.
As you can see, when photographing a landscape with distant mountains, the point of focus is not the mountains but somewhere about 1/3 in to the scene. However, estimating where this point will be is difficult, so photographers use portable aids to assist. These can be printed tables (downloadable for free from the internet) or smartphone apps such as DOFMaster or HyperfocalPro. You simply use your lens focal length and aperture to obtain the point of focus. If you don't have these handy, a rule-of-thumb is to focus on a point about 1/3 into the scene
Note that using the Hyperfocal Distance is simply a guide. Some scenes with a bland, hazy, cloudy background such as a seascape require the foreground to be really sharp while sacrificing some clarity of the far horizon. Concentrate more on getting the important subject in your photo pin sharp and then the depth of field correct.
I hope this helps... :)