An end user is the person that a software program or hardware device is designed for. The term is based on the idea that the "end goal" of a software or hardware product is to be useful to the consumer. The end user can be contrasted with the developers or programmers of the product. End users are also in a separate group from the installers or administrators of the product.
To simplify, the end user is the person who uses the software or hardware after it has been fully developed, marketed, and installed. It is also the person who keeps calling the "IT guy" with questions about why the product isn't working correctly. Generally, the terms "user" and "end user" mean the same thing.