If you want different versions of an image, such as with different crops or both a black and white and color one, you can create virtual copies in Lightroom. These aren’t duplicates of your file — you will still just have one file on your hard drive, but multiple sets of Develop instructions for that file. To create a virtual copy, right-click on your image and select Create Virtual Copy. In the filmstrip you will see that the virtual copy has a turned-up page corner (if your filmstrip is big enough), and that Lightroom automatically stacks (links) the two together.
You probably already knew all that, but did you know that you can name virtual copies?