Your e-mail address consists of several parts, each separated by a period. An e-mail address works just like a normal postal address. From left to right, it moves from most specific component to the most general.
Postal addresses do the same listing specific persons, street address, city, state, and country.
In your case the site is NC State. NC State's domain is edu, as are all educational institutions. Unless you're sending messages to another country you'll probably never use or see this. Often there will be more than one part to the site.