Cropping images can be one of the most effective ways to totally change the focus point of your pictures. It can change a totally ordinary scene into one of interest and full of energy.
Many of my most successful images have been cut from photographs that I had discarded due to their lack lustre. It's a great way to teach your creative eye to see things that aren't initially apparent and to give your photography art a much more powerful prospective.
Sometimes an image may only need a small amount of snipping to remove unwanted items from a scene, other times you can dramatically cut whole chunks away to reveal a much better focus for the image.
This video is an excellent tutorial and is well worth watching all the way through. It will help to show the different ways cropping can be used to create fantastic, striking images.
This video from 'fubdog' shows how cropping can be used to create fantastic, striking images.