When rooting android device, what is changed in device?
Although this article is somewhat old (2011), it goes over the concept of rooting and how rooting is done. Rooting is basically exploiting a security flaw (in phones which do not have have it exposed to public unlike the Nexus line of devices) to get the phone to execute code not from the manufacturer.

http://seasonofcode.com/posts/how-rooting-works-a-technical-explanation-of-the-android-rooting-process.html