Serial number provides some interesting information about your iPhone, like the week it was manufactured, the factory id and much more.
The first step is to locate the Serial Number. The easiest way to find it is to open your Settings app and navigate to General -> About -> Serial Number. You should also see the Serial Number in the Summary tab in iTunes when your iPhone is connected to the computer.
Serial numbers come in the form AABCCDDDEEF
> AA = Factory and machine ID
> B = Year manufactured (simplified to final digit, 2010 is 0, 2011 is 1, etc)
> CC = Week of production
> DDD = Unique identifier
> EEF = iPhone model, color of device and size of storage
VR0 (iPhone 2G Silver 4GB) WH8 (iPhone 2G Silver 8GB) 0KH (iPhone 2G Silver 16GB) Y7H (iPhone 3G Black 8GB) Y7K (iPhone 3G Black 16GB) 3NP (iPhone 3GS Black 16GB) 3NR (iPhone 3GS Black 32GB) 3NQ (iPhone 3Gs White 16GB) 3NS (iPhone 3Gs White 32GB) A4S (iPhone 4 Black 16GB) A4T (iPhone 4 Black 32GB)
E00 (iPhone 4 White 32GB) - Courtesy IyonUK
For example, the serial 79028XXXA4S is from factory 79 (presumably Foxconn), was manufactured in 2010 in the 28 week, and is a black 16GB iPhone 4.
Unfortunately, Apple has changed the serial number generation with the CDMA iPhone 4, so this is not applicable for the CDMA iPhone 4 and iPhone 4S.