Zip Codes vs. ZCTA
ZCTAs are not ZIP Codes, but close.
Overview
ZIP Codes are created by the U.S. Postal Service and ZCTAs are created by the U.S. Census Bureau to approximate a ZIP Code in order to provide demographics at the ZIP Code level, since many businesses conduct marketing based on ZIP Codes.
The problem is that ZIP Codes and ZCTAs don't cover exactly the same area. Here's why ...
ZIP Code Boundaries
The U.S. Postal Service defines ZIP Code areas for their own purposes based on the route that the carrier takes to deliver mail. These areas can change over time as new sub-divisions get created, or the USPS re-routes mail.
The USPS does not create ZIP Code Boundary maps. Private companies do that, based on the USPS address data. Where the lines should be drawn can be subjective, especially in rural areas.
ZCTA Boundaries
Starting with the 2000 Census, the Census Bureau created what they call a 'ZCTA' which is short for 'ZIP Code Tabulation Area.' The lowest level that the Census tabulates their data is called a 'Census Block.' For 2010, it averages out to 30 people per block.
A ZCTA is formed by grouping the blocks that fall within the ZIP Code boundaries.
Here's a video from GreatData.com which describes this well:
Want more details? Here's a ZCTA brochure, and click here for an even more in-depth explanation of ZCTAs.