OpenGl is just a software interface. It provides a standardised method for programmers to access the the graphics cards features.
Now the manufacturers in this instance nVidia provide the libraries which contain the implementations of this interface. This implementation takes care of all the "accessing the correct new features" stuff.
for example if there was a function called <b>OpenGLDoStuff</b>, the programmer will just see <b>OpenGLDoStuff</b>, no matter which graphics chip he (sorry, he/she) was using.
but the actual "stuff" it will do will vary from chip to chip, though producing similar results. In old graphics libraries cards this may involve calling the system processor, or in GeForce 1&2, the T&L and in GeForce3 libraries - the new shaders etc.