I have a few PSD files from. I do not use windows, nor do I wish to spend the money to buy Photoshop. The GIMP however can open and edit PSD files. How to I take the PSD file and make it into a full fledged working website? How do I create one from scratch?

Spacemanspiff

Set up the framework of your site in the HTML and (hopefully) CSS.
Slice the Photoshop image and save the pieces as JPG or GIFs to fit the places needed in your html.

So in other words, PSD has no point but to be able to see what the finished product will look like? It has no more value than concept art?

Of course it's useful, and if you can write the HTML and CSS properly, then it can look JUST like your PSD mockup. I do this literally EVERY DAY. I create the designs in Photoshop and present them to the client. They choose a design, and I create the site.

This site was done exactly as I described it to you Sanoms Free Edge Salon and Spa
That is exactly the way I designed it in Photoshop.

A psd file is a Photoshop document. It's amazingly useful in ways that have nothing to do with the web. Everything I expect to print at some point during the rest of my life is saved as a psd file, with layers, generally 16 bpp, and so on.