Polygons are made up of triangles. They are the simplest form of something you model with normally.

Nurbs, are made up of points that describe curves. The curves form a mesh. The curves are calculated based on the points.. so like no matter how close you get to a curve it will always be smooth. So Nurbs are basically guide lines.. and the calculations decide how to make it out of triangles.. This is why with nurbs you can change the quality of them. Ei tell it to make it out of more triangles than what it was, so that the object appears even smoother..

This might give you some idea.. I'm sure there are others out there that can explain it better than me..