Wal-Mart (NYSE:WMT) may generate more than seven times the sales of Amazon.com (NASDAQ:AMZN), but there's a good reason that the world's largest retailer is beefing up its online presence. 

In this video, longtime Fool contributor Rick Munarriz explains why Wal-Mart is getting its actual stores and regional warehouses into the act of online fulfillment in a bold attempt to take on Amazon before it starts delivering groceries -- and more -- to your door.