Composition is the way that contents within the frame are arranged or laid out. It is what you choose to include in the frame and where it is placed. Composition is a concept that is applicable across artistic mediums.
The most widely known compositional rule is the "rule of thirds." When using the rule of thirds, the frame is divided into thirds from top to bottom and from left to right which creates a grid of thirds. Placing subjects on these 'thirds' lines is an easy way create an image with an interesting composition.