A NURBS Surface is a mathematical model representing a surface. Use this tag for any questions pertaining to the "Surface" Object type in blender.
Non-uniform rational basis spline (NURBS) is a mathematical model commonly used in computer graphics for generating and representing surfaces. It offers great flexibility and precision for handling both analytic (surfaces defined by common mathematical formula) and modeled shapes.wikipedia
The Surface object type can me accessed from the add menu shift A > Surface.
Documentation for NURBS Surfaces on the blender wiki.