ISO in photography indicates how sensitive the pictures are to light, it scales usually in hundreds. The lower the number of ISO, the finer the grain produced in the picture taken. The lower the ISO, the less sensitive the camera is to light and the more elegant the grain is as well. In a darker environment a higher ISO is more appropriate to get a faster shutter, vise versa.Using different ISO setting will give different results in noise and grain.For some cameras that are not manual will automatically select your ISO for you, mainly keeping it low between the hundred and four hundred range. Which will result in the ability to shoot higher shutter speed and or smaller apertures. ISO is an important to digital photography and learning to manipulate it to get more control over your digital camera is a part of a photographers job. For a more in depth introduction of ISO click here.
Aperture is measured in F-stops, when pressing on the shutter button your camera opens a hole which allows some light to get in from the scene. The larger the hole, the more light gets in ranging from f-stops as big as 1.4 to f-stops as small as 22 etc. What confuses amateur photographers is that more light is received than is needed or intended, this is because the smaller the number in f-stop, the more light comes in. This is why moving the f-stop to the next doubles or halves the amount of opening in your lenses. Depth of field is the amount of the image that will remain in focus, whether the camera is far or close to the subject. On F-22 everything is being focused, while in F-8 the background is a bit more blurry than the rest of the picture. For more understanding of depth field and aperture an introduction can be found here.
The amount of time the shutter is open for is the shutter speed. Measured in seconds making 1/3000 faster than 1/50 the amount of time exposed the more it captures, for example movement of a person or any other type of movement. Sometimes adjusting shutter speed to needs is whats necessary for the perfect picture. Although, recommended that keeping shutter speed fast, using a slower shutter is okay depending on the environment for example water flowing would be a better picture if using a slower shutter to give off a dramatic affect. It all depends on the scenario and other aspects of the exposure triangle if all line up perfectly with each other, it makes for the perfect picture.