What do those "sell-by" dates on food mean?

Have you ever wondered if it is okay or not to eat food that is older than it's "sell-by" or "best-buy" label?  An article in the Washington Post, "The 'sell-by' dates on your groceries are uselss. Here's why.", explores this topic. It seems many of these labels are largely for use by the retailers, and shouldn't prevent customers from enjoying the food products after their package "sell-by" dates have gone by.