Understanding shutter speed and aperture is key to getting out of Auto mode and creating the photos that you want to take. Luckily, these tools aren't anywhere near as scary or complex as they appear.
In short, this describes the amount of time that the camera's shutter is open.
The shutter covers the camera's sensor. As you take a photograph, the shutter moves to reveal the sensor. The amount of time that the sensor is exposed is what we call the shutter speed.
So what is the difference between a slow and a fast shutter speed?
Slow Shutter Speed
A slow shutter speed (usually considered as anything below 1/50th of a second) means that the shutter reveals the sensor for longer. This allows more light to hit the sensor, over a longer period of time, which results in a brighter image.
The main reason to use a slow shutter speed is that it introduces motion blur into the image. This is because the camera is taking a photo for the entire duration of that slow shutter speed, and within that time period your subject will probably have moved position. Choosing a slow shutter speed can be a creative way of conveying movement, although it can be tricky to use at first and requires lots of practise to get interesting images.
Fast Shutter Speed
Most people, certainly to start with, choose faster shutter speeds (considered 1/100th of a second or higher). This means that the shutter reveals the sensor for a shorter time, allowing much less light to reach the sensor, resulting in a darker image.
The main reason to use a fast shutter speed is to freeze motion. If you want to guarantee that the subject is completely sharp and frozen in your photo, use a shutter speed of at least 1 over the focal length of your lens. In other words, if you have a 300mm lens, use a shutter of at least 1/300th of a second. The faster the subject you are photographing, the faster you will probably want your shutter speed to be. This will not only freeze the action but also reduce camera shake. A tripod is handy for steadying the camera, especially with slow shutter images where camera shake can ruin an image.
The aperture describes the size of the physical hole within the lens. This is measured in f-stop numbers.
A low f-stop (for example f2.8, f5.6) describes a wide aperture. This lets in more light, allowing for brighter photos. You can increase the shutter speed now to darken the photo if necessary.
However it also means that the depth of field in the photo is smaller. In other words, less of your photo will be in focus. This is good if you want to isolate your subject from the background. It also creates soft pleasing tones, like in this photo below of a prairie-dog.
A higher f-stop (e.g. f11) describes a small or narrow aperture. This lets in less light, allowing for darker photos. You can decrease the shutter speed a little to let in more light.
In contrast to the wide aperture photos, the depth of field is now larger, meaning that more of the scene is in focus. This is good if you want to photograph landscapes or give more of a sense of context.
Play around with the f-stop until you are happy that everything you want to be in focus in focus. Is your aperture too wide or too narrow? In the photo of jack-rabbits below, a narrower aperture (higher f-stop) would have resulted in both rabbits being in focus. Using a wider aperture and focusing on the rabbit in the foreground has resulted in the background rabbit being blurred.
Something else to bear in mind is that the closer your camera gets to your subject, the smaller the depth of field gets, and the more you will have to increase your aperture to get more in focus and to let more light in.
A slow shutter speed lets in more light and adds motion blur.
A fast shutter speed lets in less light and freezes motion.
A wide aperture results lets in more light and results in shallower depth of field (less in focus).
A narrow aperture lets in less light and results in a greater depth of field (more in focus).
Getting a balance of these will result in the correct amount of light (the correct exposure) for your photo.
So you have the shutter speed and the aperture that you want, but you're not getting enough light on your sensor, meaning that your image is too dark. So what do you do? You reduce one or the other to let in more light. But sometimes this isn't an option - for example, sometimes lowering your shutter speed a little will result in more blur, which you might not want.
This is where the third key component of the camera comes in: ISO. The higher the ISO value is, the more sensitive the sensor becomes to light. On your camera, this will be represented as ISO 100, 200, 400, 800. If you need to brighten your image, without changing your other settings, then increase the ISO. The one drawback is that the higher the ISO, the more digital noise in your image, which can sometimes degrade the quality of a photo.
I photographed this raven one evening with a fast shutter speed (around 1/2000th of a second), to freeze the bird's fast flight as it flew by me. I therefore used a fairly wide aperture (around f8) to let some more light in. It was important to not go much wider as less of the bird would have been in focus (notice how the wing tips are out of focus). However, as it was evening, my image was still not bright enough, so this is when I increased the ISO. There is a little bit of digital noise (the reddish dots on the sky), but in my opinion having this sharp photo is better than no photo at all.
Which Camera Mode Is Best?
Aperture Priority (A or AV mode, depending on your camera brand) allows you to choose the aperture and ISO. The camera will adjust the shutter speed itself to correctly expose your image.
Shutter Priority (S or TV mode, depending on your camera) allows you to choose the shutter speed and ISO, and the camera will adjust the aperture size to correctly expose your image. A drawback of this is that if you pick a very fast shutter speed and low ISO, then the camera is unlikely to find a wide enough aperture to let in the light you need for a correct exposure, resulting in many missed photo opportunities.
Full Manual Mode (M mode) gives you full control over shutter speed, aperture and ISO. This is useful for taking test shots but it requires much more work and fiddling with camera settings, taking away your concentration from getting the shot. You may find it hard to adjust settings quickly enough as the situation changes.
I recommend Aperture Priority for ease of use and reliability, and rarely will you need anything else. More information on this can be easily found online and it is a case of trying out modes and settings and seeing which you prefer to use.