Entrepreneur.com has an overview of the practice here that should help in a general sense. Is there any specific aspect of the market you are interested in knowing about?
This is the practice of selling goods without housing any stock yourself. You pass the order to your supplier who then ships the item directly to the customer. So you charge one price to the customer and the supplier charges you a lower price and you pocket the difference. I have some experience of this with eBay sales and had a small business based around it. That said, unless you have really reliable suppliers with excellent communication it is easy to get into problems quickly, which is ultimately what I found to be the problem. Used as a part of a business rather than the whole business it could work well.