This is The Phoenix Project RPG, a completely free, superhero expansion pack for d20 Modern. These rules layer on top of Modern with relatively few changes. If you know how to play Modern or D&D 3.X, then you can play this game with relative ease.
The site is separated into pages that represent chapters in a book. The first six of them—Character Creation, Classes, Skills, Feats, Powers, and Gear—provide all the information you need to make a character. They’re in order of how fundamental they are to your character. Ability scores are the most basic stats you have, on top of which go skills, for example. The last three pages—Combat, GMing, and the Appendices—segue from game rules for everyone (Combat) to specific guidelines and tools for running the game (GMing and Appendices).
That being said, don’t ever think of Phoenix as something that you have to read end to end in strict, linear order. It’s a game, not homework. The whole point of arranging this information on a website is so that you can consult it, access it like a manual, so in addition to pages, the rules are also organised by “tags,” which are key words that appear in the individual entries on the site. You can see the tags displayed in a cloud in the right-hand menu.
The difference between the two is that pages are strictly hierarchical and tags are messy and horizontal. If you want to know everything that goes under “combat,” then look at the combat page. If you want to know every entry on the site that mentions the Stealth skill, then click “stealth” in the tag cloud, and a dozen posts will appear.