Removing an unnecessary picture border or cropping out part of an image is one of the most commonly performed photo-manipulation operations.
To select the part of a picture you want to crop to, use the Rectangle Select or Scissors tool.
The Scissors tool allows you to stick to an edge when selecting part of a picture. You can then cut out what's inside the edges you have selected.
Cutting out an interesting part of a picture.
Selecting a panorama from a square photo.
The Rectangle Select tool lets preview what you've selected from your image before cutting it out.