and What products are available through it and which aren't?
The SRD? Not very often, these days.
The PRD on the other hand, gets updated when the community team updates it, and contains all rules from the Rulebook line, plus a couple more.
There's a link just to the left of that devilishly handsome dark elf, too.
If you follow Chemlak's link to the PRD, you'll see the list of included books on the left: Core Rulebook, Advanced Class Guide, Advanced Player's Guide, Advanced Race Guide, Bestiaries 1 thru 5, Game Mastery Guide, Monster Codex, Mythic Adventures, NPC Codex, Unchained, Occult Adventures, Ultimates Campaign, Combat, Equipment & Magic, and Technology Guide. The next book added should be Horror Adventures, but I don't know when it will happen.
If you're talking about d20PFSRD.com, it includes all Paizo Pathfinder books, leaving out Product Identity like proper nouns, and many third-party products as well. It generally adds books pretty quickly after release, but it's not run by Paizo.
thanks for the help.