as I understand it, it is same function that takes sensory information from outside world and builds models, sees patterns and cause-effect relationships
only Ni works to rearrange the model of the world you have inside your mind giving you ability to see reality from multiple perspectives
and Ne works with the world outside of yourself and gives you ability to see how reality can be re-arranged to be, future potential and possibilities of this reality
Ni takes sensory data and goes "may be it is this way, or this way, or that way, or may be this way"
Ne takes sensory data and goes "may be it can be made to be this way, or that way, or this other way"
The best description of Ne that I have ever read was on 'greenlightwiki'...'Lenore Thompson'. But that site is unavailable at the moment, for some reason. Simulatedworld's post about the function attitudes is also very good.
Since I can't link the previous site which goes into more depth, I have a post in another thread in which I quoted parts of it and elaborated on it. It should give you an idea of what Ne is.
I don't really understand Ni, though...InvisibleJim's article on 'Ne and Ni interactions' was what just started making me understand Ni a bit more. But I'm still not completely confident in my understanding of it.