Stock photography is the sale of images and illustrations for use in advertisements, magazines, fliers, television, etc. You get the idea. Stock buyers usually are in the market of selling or promoting things, but are not often capable or have interest in taking the pictures themselves. That's where the photographers come in. There are stock photography sites that will sell your images for you, while you make a small profit. I've been accepted to a few, and have been having a good time improving my skills.
Shutterstock for Photographers - This is where to go if you're interested in selling your images and making a few dollars
ShutterStock for Designers - This is where to go if you're interested in buying images
My ShutterStock - This is my gallery of images for sale at ShutterStock
iStockPhoto - Sells images per picture, instead of as a subscription
My iStockPhoto - This is my gallery of images for sale at iStockPhoto
Dreamstime - Also a per picture sales site
My Dreamstime - My gallery at Dreamstime
BigStockPhoto - Another per picture sales outfit
My BigStockPhoto - My gallery at BigStockPhoto