E-tickets are tickets that you order online, but you don't get your ticket until you arrive at the park. Instead, you get a code after you order the tickets. Then you go to the kiosks, type in your number, swipe the credit card you used to purchase the tickets, and the kiosks print out your tickets. Universal charges $2 for this service. It's fast and pretty easy.
I don't recall e-tickets having an expiration, but does anyone else know more?