Free Sites are typically promotional sites for a membership sites, and have a fairly consistant structure about them. You will usually find an index or entrance page, followed by a welcome page that has links to 2 or more galleries for the site on it. Each of the galleries is typically similar in nature to those you would find on a TGP, although sometimes having slightly fewer pictures on the page (a TGP site might have 15-20 pictures per page, where a free site will typically have 10 - 15 per page).
Some free sites also have movie clips or a hybrid combination of movie clips and still images.
There are also free sites for erotic stories.