If you have a new digital camera, or if you’re new to digital photography, all those abbreviations on the top dial of your camera might seem a bit confusing. Your top dial is where you will find your camera’s exposure modes.
Contrary to popular belief, the exposure modes you shoot with aren’t a reflection of your technical ability. Your exposure mode of choice is also about selecting a mode that gives you the freedom to stop worrying about other settings and start concentrating on taking great shots.
Your digital camera will offer a number of automatic settings, including modes that help you to shoot action, close-ups and portraits, but these shooting modes can be restricting and should generally be ignored. Get to grips with your camera’s semi-auto and manual settings and you’ll soon see your shots improve.
This is the ideal mode for complete beginners. The D-SLR is practically converted into a compact point-and-shoot, with exposure settings, aperture and shutter speeds all taken care of.
2. Program
Here, aperture and shutter speed are set automatically. However, you control ISO, Exposure Compensation (ie, going lighter or darker) and other settings. You can override the D-SLR’s suggested settings if you wish.
3. Aperture Priority
This semi-automatic mode enables you to choose an aperture value for your desired effect (blurred backgrounds, for example), and the camera then selects the shutter speed that’s needed for a correct exposure.
4. Shutter Priority
This mode is similar to Aperture Priority, but you select the shutter speed you require and the camera takes care of the aperture. This is perfect for freezing high-speed action by choosing a fast shutter speed, or for creating motion-induced blur using a slow shutter speed.
5. Manual
In Manual mode, you set both the shutter speed and the aperture for any given scene, which places you in total creative control. You’ll now have access to all of the available shutter speeds and aperture values, and can also use Bulb mode. This additional mode enables you to shoot exposures for as long as the shutter button is held down, and is ideal for night photography.
source
http://www.digitalcameraworld.com/2016/03/21/a-beginners-guide-to-your-cameras-top-dial/