Arcanis a campaign setting for D&D/Fantasy d20. It is a somewhat dark-fantasy world, not necessarily in look, but definitely in tone. Empires are constantly at the edge of world-spanning war (last I checked, it truly was coming), demons are always waiting at the edge of your vision, and there is enough political intrigue to make Julius Caesar look like a baby in diapers. The setting is heavy on story, and is made for heroes to make their mark, either for better or for worse.
Check out 
www.ParadigmConcepts.com for basic info on the products, 
www.OnaraOnline.org for the official unofficial fan site (with tons of history, background info and story material), and 
www.RPGA.com for info for playing in Living Arcanis, the largest 3rd-party Living campaign (if you are a Herald-level DM, you can order Living Arcanis adventures free and play them at home).
The PCI folks are not necessarily all the time on these boards, but they do check from time to time, if you want more info. You can also try their Forums at the PCI website.