NURBS are the Non-Uniform Rational Basic Spline. NURBS 3d modeling is a complete term of 3d modeling in NURBS 3d modeling curved faces and edge and vertex are modeled using NURBS modeling tools and commands in Maya and other 3d computer graphics software applications. In this 3d modeling there is two way to start 3d modeling of any object. One is using the NURBS primitives i.e. starting with NURBS cube, NURBS sphere, NURBS cylinder etc. and other one method is NURBS 3d modeling using the CV (control vertex) curve tool and EP (Edit Point) curve tool.
NURBS primitives:
NURBS primitives are basic 3d shapes which are edited using tool and command, these commands and tools can perform their function at non uniform the curved faces and edges and lines, and vertex point which are irregular as compare to polygon modeling. 
If you want to draw a curve on an object’s surface first you have to make its surfaces like a grid and for that purpose you have to select the object on which you want o to draw curve and then click on the magnet icon tool this changes your image mode to grid and then you can draw a curve. To return back to the previous mode click on the same magnet like icon tool.
CV Curve tool:
his is a tool used in 3d NURBS modeling, what can you do with this 3d modeling tool is just you can draw the 2d curve line but the line can only be drawn on grid, if you want to draw a curve in standing positing then hit space bar go to the front view port and there you can draw a standing curve. Then you can return back to the perspective view port. For drawing a curve on 3d objects you have to first change them into the grid using the ‘Make object surface live’ which is a tool with a magnet icon. The procedure is told in last paragraph.
Duplicate surface curve tool is the tool at the top of menus in Edit curve menu. This tool is only for the surface and not for grid and 2d line you can duplicate a curve present on a surface with this tool. For more details watch this free tutorial video of the introduction of the 3d modeling NURBS and many other videos are also available here for more complete comprehension of the 3d modeling.
