The $, $$, and gold coin signs are to show donator status. They can all be purchased through the NS store. People who have an extra couple bucks to toss into the site or people who have wasted their lives on here. You get access to the donator forum as well but that's been fairly slow these days.
If you like NS and want to support, and get that $$$ to separate you from the other users do it up. I figured I spent enough time on this website I should just throw down and grab the coin. Yolo and stuff.