One of my favorite camera controls is the aperture setting. Aperture controls your depth of field and the portion of the photograph that will be in the sharpest focus. Do you want a photograph with crisp clarity throughout or a narrow band of focus on the main subject and increasing blur on the rest of the image? Your decisions on the camera and lens set up will determine the way your photograph looks. With practice, it becomes second nature. Many beginning photographers try to create an image with edge-to-edge sharpness because it seems logical that you'd want that. Their primary experience with lack of focus comes from motion-blur-ruined photographs that they took with their point and shoot cameras, with their arms extended to see the viewfinder (an inherently unstable stance, contributing to the motion blur). Also, since they are usually shooting in Program mode, the camera solution tends to apply as much depth of field as possible. This gives the new photographer the greatest chance to get his subject in focus and be happy with his new purchase. You top that off with a kit lens that has a very limited ability to open up the aperture.
As the new photographer begins to learn more about her camera and that the entire photograph doesn't have to be in sharp focus, she'll want to create photos with some out of focus areas, especially when making portraits. This is when she, also, learns a different (and much more expensive) lens is the requirement to get that look.
You can go into Manual mode and control all the settings, including aperture, or you can set the aperture setting and allow the camera to choose the other settings. This setting is called aperture priority on Nikon, with an "A" on the dial; aperture value on Canon, "Av" on the dial. This setting allows you to control the depth of field for your photograph. The depth of field is the distance between the nearest and farthest objects in a scene that appear acceptably sharp. It is expressed on your lens with f stops. There is an in-depth discussion of all this here, if you want to go beyond the scope of our blog.
The best way to learn how your lens aperture setting will impact your photos is to practice with different settings. The easiest way to remember the impact of f stops is that low numbers will create a shallower depth of field and the higher numbers will have a greater depth of field. Unfortunately, this is not quite the entire story - distance to the subject also has an impact.
Here the fence pickets are in an 8 inch by 8 inch square, shot at f2.8 (low number, shallow depth of field). The focus point is the center, rear picket, and it is in sharp focus. The other pickets are out of focus to varying degrees (based on their distance from the focus point) and the background is completely out of focus.
If you place your focus point far enough away, even an f2.8 lens aperture setting can record a large scene in focus. This photo was made with the exact same lens and aperture as the fence pickets, but is in sharp focus because of the distance between me and my point of focus.
The interaction between aperture, depth of field, and focal distance can throw you off if you aren't one to go into the physics of it all. Going to the link above, to Wikipedia, is worth the trip. But, if it all seems like magic, do not despair. You don't have to understand the science to make the effect work for your photographs. Practice with your lens settings, so that you can learn how your aperture settings impact your final photo. You'll have more fun when you can get the photographic effects you want, when you want.