The aperture is the size of the "hole" that captures the light into your camera sensor. The wider the aperture the more light that will reach the sensor in a fixed length of time.
Aperture is measures in f-stops each f-stop indicates a number that divides the focal length of the camera lens. For example if your focal length is 28mm an f2 aperture means 28/2 = 14mm of aperture.
For a 50mm focal length an f8 aperture will mean 50/8 = 6.25mm of aperture. If the focal length is fixed the higer the f number the smaller the aperture will be and less light will reach the sensor. When you use a high f number the aperture will be very small meaning just a tiny fraction of light will pass to the sensor so you will probably need longer shutter speeds to reach the right exposure in your picture.
Aperture and DoF
The aperture setting basically controls the depth of field (DoF) in your picture. The depth of field is the area in your picture where objects will be sharp. A single lens can only focus in one point at the time so as we move away from that point objects start to lose focus and will be less sharp, with smaller apertures the effect is greatly reduced so objects away from the focus point will be sharp and for very very small apertures practically all the points in the picture will be sharp regardless of where the focus is.
In some cameras you have a DoF preview button that will basically switch the camera to the selected aperture and show you in the viewfinder how things will look like. Don't worry if the image in the viewfinder is too dark because you will control that with your shutter speed, what matters is how sharp things are in the viewfinder, control your foreground and background and check if everything looks sharp, if not then you probably need a smaller aperture (bigger f number).
So for large f numbers the aperture will be small, less light will enter the camera and the depth of field will be bigger. On the other side small f numbers will produce a wide aperture with a lot of light entering the camera and a narrow depth of field. These important concepts are helpful to understand when to use each aperture setting.
Landscape Composition Apertures
In a Landscape composition you usually have some foreground like rocks, grass etc. Something in the middle and some background. The foreground and the background are usually sepparated by a long distance and you want both of them to be sharp in your picture. This means you need a very small aperture produced by a very large f-number. Apertures such as f16 and higher are used, f22 to f36 are common for this type of pictures.
Since the aperture is really very narrow the depth of field will be huge meaning everything will be sharp, you usually don't need to care about where to focus the camera and something in the middle should be good enough, if not then you probably need an even higher f-number.
The amount of light reaching the sensor will be reduced enormously by the aperture so a large shutter speed is probably needed and that is why you are going to need a tripod to make everything sharp. A remote shutter is also needed and in case you don't have one you can use the camera self-timer to avoid moving the camera when the shutter is pressed.
General Purpose Apertures
When everything you need to capture is at about the same distance from the camera you don't have to worry about different focal points and you can use an intermediate aperture to make sure your picture will be sharp without the need of a very small aperture. Apertures such as f8 to f11 are normal for these situations. It is common to say "just put your camera on f8 in aperture priority mode and don't worry about the rest" this is a practical advice and really works in general purpose pics but fails when you are in a special situation.
Portrait and object Apertures
When you take a picture of a person or an object you want that person or object to be incredibly sharp and you don't worry about the rest. For these situations very wide apertures (small f numbers) are perfect.
The smaller the f number the more reduced your depth of field will be producing a focused subject with a blurred background. This effect is called Bokeh. For very wide apertures such as f1.8 or f2 the effect is really intense with a focused object and a completely blurred background. If you need a wider depth of field you can go up to f5.6 depending on what needs to be sharp and what not. The wide aperture also means a lot of light will enter the sensor so you will be able to take good portraits and pictures inside a house without needing to use the flash.Conclusion
Understanding how the aperture and the depth of field relate is critical to be able to select the right aperture for the right situation and the desired final result. Experimenting with your camera in aperture priority mode taking the same scene with different apertures and examining the result is a very important experiment to understand how the aperture works in your camera. You have to experiment to see which aperture produces the best portraits for your camera and lens, which aperture blurrs the background, which is a general purpose aperture and how narrow you have to go for landscapes.
