Selective color is a post-processing technique where most of a photo is converted to black and white, but some parts are left in color. This is usually achieved by using layers and masks in photo editing software (Photoshop, SilverFast, Adobe Fireworks or The GIMP for example). This technique is often over-used. Use it to enhance your composition storytelling, not to merely demonstrate that you know the technique.
Here’s a link with some more great inspirational selective color photographs