ISO is how much light is let into the camera and how sensitive the image will be to light. ISO controls how bright and dark an image will be, if it is not used properly the image can end up too dark or too bright. The image can also end grainy.
Shutter speed is how fast the shutter opens, it is used to let the correct amount of light in for that image. Different shutter speeds are used for different things such as a quick shutter speed would be used for sport for a quick sharp image and slow would be used for such things as water falls to make it look like its floating, if not used correctly it can lead to motion blur.
Aperture is how much light is let in the hit the light sensor by the lens, the bigger the hole means more light is et in which leads to larger depth of field but can also lead to blur. A smaller hole used means better and sharper focus.
There are five different parts to photo composition such as rule of 3rds, the is the grid on the camera that helps you position images correctly to make the photo look natural. There also framing which is used to make an image look better as in blocks the unneeded parts of the images to focus on the main point and can also give context and intrigue the viewer.
There is also leading lines which helps you follow the image to a focus point of the image which can either be the main point of the image or a person, it can start on or off the page and can lead to something distant in the image. There is also symmetry and viewpoint, view point is when the subject is in the frame or changing the camera view point.