Taken from whatis.com:

NURBS (nonuniform rational B-splines) are mathematical representations of 2- or 3-dimensional objects, which can be standard shapes (such as a cone) or free-form shapes (such as a car). NURBS are used in computer graphics and the CAD/CAM industry and have come to be regarded as a standard way to create and represent complex objects.


Polys by geometry are planes bound by straight lines...I couldn't find anything else to describe them.

Most people after some experience using Maya, know what to use for what and why.....which I am sure our 3D guru's can answer