Framing in Composition
In the photograph to the left, taken at Swallow Falls State Park, the trees frame the composition of the staircase. The main compositional element is the S-curve formed by the staircase, but the trees add framing or context to the composition. In the photograph below, I had to crop the photograph significantly so that the Yellow-Crowned Night Heron would be large enough in the frame to be interesting. But if I cropped too severely, the bird would look unnatural. After all, herons don't fly like that; they only look like that when they are landing, so I decided to frame the shot by including where he was planning to land in the framing of the shot.
changes the relationship between the foreground and background of your image; cropping does not. Cropping simply cuts out part of your image that you consider to be less important to your composition.