Plagiocephaly is the proper term for this. You can do an internet search to find out more (otherwise this will be a novel). But from my experience (a friends child had this), depending on the severity he will likely require therapy or in other cases a molding cap/helmet to correct it. There is a lot more to it, but like I said, too much to post here. There is a lot of info online, but some of it is extreme. Don't let it scare you. Has your Dr. mentioned this at your well baby visits? Usually they will tell you when you need to be concerned about it... Best of luck. Hope you have a mild case that will self-remedy.
Here is a link to a recent post from a mom asking about the molding devices. She may be able to help you more since she is experiencing this first hand...
http://www.mamasource.com/request/recent/49684/1184591202/10