When I started up with fishkeeping ten months ago after a 15 year hiatus, lots of things in terms of the science and convenience had changed. The UGF (undergravel filter) that used to be a staple in my fish-keeping world is now apparently very old-school, and some of the newer technologies will not be compatible with that kind of filtration system.
But the basic idea, IIRC, is that the filter plates (which are nothing more than raised plastic mesh) are laid on the bottom of the tank, and the gravel laid over top. The gravel itself acts as an initial filter medium; the water gets sucked through the gravel, the crappy stuff and dirty water is theoretically trapped under the filter plates that lie beneath the gravel substrate, and fresh water is transferred up and out of the plastic posts attached at each corner of the tank from under the gravel, with a small cotton/charcoal filter in each post.
As much as people on this forum say that your local fish store (LFS) may have bad advice, it might be worth your while to go to your LFS, check out the different filtration systems they have, take notes and come back to this forum to ask for more specific directions based on what you envision doing with your new tank.