There are a few common reasons why chains can slip:
1. Most likely guess - your cable has stretched - you need to retension it
2. Your chain is worn (as though stretched) and needs to be replaced
3. The derailleur hanger is bent (LBS can straighten it if you don't have the tool)
If you have under 1.5K miles, it's not likely that your chain is worn, but you can check this.
The derailleur hanger is a a piece attached to the frame that bends so that your frame won't. If your bike has fallen over and gotten banged on the derailleur, this is possible.
Go to the Sheldon Brown website that jazzy cyclist pointed at, and it will give clear explanations of how everything is supposed to work, and all the jargon people will use on this site. If there are some points that you have trouble following, ask on the "Bike Mechanics" section here. Best of luck!