Aperture is apart of the exposure triangle and occurs in the lens rather then in the camera. Using aperture is equivalent to the pupil of an eye because it can affect how much light is involved. Using a wide aperture such as f/1.4 brings more light into a dark room. Aperture is also used for Depth of Field. If you want a shallow depth of field, where the background is out of focus, then you'd need a wide aperture such as f/1.4. If wanting a deep depth of field, where there is lots of detail all through out the photo, then use a small aperture such as f/22 as well as a tripod.
Shutter Speed
Apart of the exposure triangle, shutter speed is a way to play with motion in the frame. If there is a moving object in the photo, then having a quick shutter speed will be able to capture the moment without the image being blurred.
ISO
ISO stands for the International Standards Organization and it affects the sensitivity to light. A high ISO makes an image brighter but could lead to grain. Typically the ISO should remain at 100 and if the exposure isn't how the viewer want's it then the ISO could be brought up. ISO is also apart of the exposure triangle.
White Balance
The white balance keeps the white in an image it's true white. It's a way to adjust the colors to make the photo look more natural.
Contrast
Contrast is the brightness and saturation levels in a image. High contrast means there is a distinct difference between black and white. While low contrast is mostly the in between gray.