Depth-of-field in photography is easy to understand. It refers to the area of a scene which is in focus. This may be the foreground, the middle ground, the background or the entire image.
Start in the foreground and move through a horizontal plane towards the background. If the depth of field is large then more of the scene is in focus and if it is small then less is in focus.
Above: Tiny Door Bell by Anne Darling
Simply put, the smaller the aperture, the greater the depth to your picture. Smaller apertures are obtained by using bigger numbers. For example, f/16 will give you a smaller aperture than f/4. So f/16 is a better choice if you want to include more of the scene in the background. Landscapes usually require a smaller aperture as we want to see the whole scene.
Remember that if you change your aperture by one stop, say from f/8 to f/16, then you need to change your shutter speed by one stop too. So if your aperture is one stop smaller, letting in less light, then your shutter speed needs to be one stop slower to compensate. Trial and error is your best teacher here, and lots of practice.
Landscapes, buildings and other inanimate objects are good for practising the effect of changing your aperture because they are subjects which won't move much. Once you have mastered it, you can move on to do portraits, wildlife and so on.
In the photograph above, I wanted the bell to stand out as the main subject so I chose a shallow depth-of-field (large aperture), leaving the foreground out of focus which gives a blurred area known as bokeh.
The term bokeh is often used in macro photography where depth of field is usually very small. A depth of field that is very small means that most of the image is blurred and out of focus but a small range, or depth, remains in focus.
Using a zoom or telephoto lens may also produce bokeh due to shallow depth of field.
In photography depth of field is inversely proportional to the focal length of a lens, so if you are buying a new lens remember that, for example, with a 28mm lens you can potentially get more of the picture in focus than with a 100mm lens.
The principle aim of bokeh is to reduce distraction, usually from the background, but in this case I have used it to reduce distraction in the foreground and to create a frame for the tiny bell.
Bokeh also has an aesthetic meaning when it refers to the quality of the out-focus-areas. Good bokeh occurs when the edges of the out-of-focus objects are indistinct. Bad bokeh occurs when the edges are still defined.
If you want to know more about depth-of-field you need to read up on 'exposure'. Follow the links for my recommendations.