POSTNET is a Postal Numeric Encoding technique that's used to sort mail for delivery, using the destination zip code. You've probably seen it on certain printed envelopes. It's a very different type of barcode than the ones that are used for retail or other data storage purposes. They're considerably shorter in height, but generally much longer in length than the traditional ones.
Specifications: Unlike most barcodes, which encode characters using bars and spaces of different width, POSTNET barcodes are encoded based on the height of the bars. They use a two out of five barcode, which means that each digit is encoded as five bars, two of which are full height. POSTNET barcodes can be encoded as five digits, six digits, nine digits, or eleven digits. The last option includes the entire 9-digit zip code (known as ZIP +4), plus the two digit delivery point code.
Advantages: The POSTNET barcode includes a check digit to ensure accuracy.
Disadvantages: Since it takes five bars to encode each character, plus one five digit check digit, as well as start and end bars, the POSTNET barcode is more than five times as long as the information that it's encoding. It also only encodes numbers, not letters or other characters. The code is, in fact, largely outdated in general, and is being phased out of use in favor of the Intelligent Mail barcode.
Intelligent Mail - A more detailed and efficient method of encoding postal information, the Intelligent Mail barcode includes the sender's information as well as the recipient's.