The depth of field has a few variables which affect it. Your camera, your aperture setting and the viewing distance (how close or far you are from the object you are trying to photograph). There is a gradual change from sharp to unsharp within the DOF. The in focus region is the DOF. Then there is Bokeh, from Japanese (pronounced bo-ké), it is the quality of the out focus region. When someone tells you have good Bokeh, they are saying that section of the image that is out of focus is harmonic, well blended and smooth looking.
 
All objects at the point or distance of focus will be sharp, those out of range of the point of focus will be not sharp, in varying degrees from the point of focus. A large DOF will mean that most or your entire image is in focus or sharp and a small or shallow depth of field will mean you are limiting what areas surrounding the object of focus are sharp. Controlling which parts of an image you want to appear in focus and which parts that are not in focus, can mean the difference between the failure and success of an image.
 
All right, so how do we control DOF in an image? Aperture settings of course and these are measured in f-stops. Depending on the lens, the range of aperture settings can be anything from f/1.4 to an f/64 (also known as f/stops). These settings refer to the size of the opening on the lens which allows light to enter the camera. We use f/stops as the ruler for our aperture settings.
 
And just to confuse the beginner further, it’s the exact opposite of what one would expect, an f/2.8 aperture setting is a larger opening on the lens than the f/22 aperture setting. A small aperture (f/22, f/32, f/64) setting allows limited light to enter and a large aperture (f/1.4, f/2.8, f/5.6) setting allows extensive amounts of light to enter the camera. An f/stop of f/2.8 is half the size of an f/stop of f/1.4, each time you change to a smaller f/stop (larger number) you are reducing the opening of the lens by half again as much.
 
So we can let in more light, what else does the setting the aperture do for our images? It controls the Depth of Field. A small aperture setting of say f/22 will increase the range of sharpness surrounding the object you are focusing on. If you want to blur the background or create a shallow depth of field around the object, you would use large aperture setting or an f/stop of say f/5.6 or less.(And by less, I mean an f/4 or an f/2.8 or an f/1.4)
 

Aperture Settings

This chart shows an aperture setting of f/2.8 which lets in more light to an aperture setting of f/22 which lets in much less.
 
Examples:
If you change the aperture setting from and f/stop of f/16 to an f-stop of f/8 it is TWO STOPS brighter.
If you change the aperture setting from an f/stop of f/5.6 to an f/8 it is ONE STOP darker.
 
As you can see by these two examples, aperture settings not only affect the depth of field but they have a significant relationship to exposure.
 
In Landscape Photography, more times than not, you will want the entire image to be in focus therefore you would want to use a smaller aperture setting, say of f/11 or f/22 or smaller. In Macro and Portrait photography the opposite is true, most times you will want the background blurred or you would want to use a larger aperture setting or an f/stop of f/2.8 or an f/4 because by blurring the background you are directing the viewer’s attention to the object in sharp focus.
 
Mostly likely when shooting portraits you will want to use settings similar to those used in Macro photography, in other words larger aperture settings because your focus should be the person you are shooting. I love the zoo, but taking pictures of the animals in some of the surroundings created for them detracts from the animals themselves, so I would recommend using a mid-range to large aperture setting, say an f/4 through an f/8, which would blur the background and remove focus from those unwanted objects in the background that might draw attention away from your focal point. These are meant to be guidelines to help you to learn to control your aperture settings and the DOF of an image.
 
Each scene or object will demand you evaluate it and choose the appropriate settings based on how you visualize the final image.
 
Three examples, aperture settings of f/6.3, f/11 and f/22, respectively:

Aperture f/6.3

Aperture f/11

Aperture f/22