Rocs seem confusing, but it's not really all that bad to understand.
The first Roc came out about '87. It was a small diameter 21.2 cm diameter disc based off the XD. In '89 that disc was replaced with the first version of what we consider a Roc today-the 21.7 cm diameter San Marino Roc. This disc quickly became the most popular disc on the market. At the time the Stingray was about as long a disc as you could get, so the Roc was used by many players for almost every shot.
In '94 or '95, something happened to the Roc mold and the disc was replaced with a new version. Innova's mailing address had changed, so the tooling on the new disc said "Ontario." Thus the old disc became the "San Marino" or "Sanny" Roc and the new disc was called the "Ontario" Roc. The Ontario Roc has a higher dome and flips easier than the San Marino's. Because of this they were not very popular and they were replaced in 1997 by the current "Rancho" Roc, so named because Innova moved from Ontario to Rancho Cucamonga and the tooling once again changed. The Rancho Roc is the most overstable Roc yet, and has been hugely popular.
So, there are really only three molds: San Marino, Ontario and Rancho. If you go to the store and buy a DX Roc or a KC Pro Roc, it is a Rancho mold. From most overstable to least overstable they go Rancho-San Marino-Ontario.
When San Marino's and Ontario's were in production, there was only DX plastic. All plastic variations of the Roc such as KC Pro Rocs, Super Rocs and USDGC Roc are Ranchos. The exception are a few collectors USDGC Rocs that have been run in the Ontario and San Marino molds the last two years, but if you don't understand Rocs and you are paying collector USDGC Roc money for a disc...well, you need more help than a post on a message board can give you.
Last year the DX Ontario mold was brought back as a CFR disc. This year they have re-created the DX San Marino Roc in the CFR program and added an Ontario Roc in Star to the CFR program. So you can find some new San Marino's and Ontario's now with tournament stamps. Any Roc you find with he standard factory stamp is still a Rancho. You can still tell the difference by flipping them over and reading the tooling on the underside.
The original 21.2 cm diameter disc came back in '95 as the "Classic Roc." It has the "Roc" name, but it should be considered a completely different unrelated disc to the other Roc's. It is nothing like a Roc.
That's about it. Actually, it is not...but basically that is it.
Edit: This thread came back from the dead, and the saga of the Roc continues. In 2009 they put the recreated San Marino mold into regular production in Star plastic. In 2010 they introduced the Roc+ mold, which features a slanted inner rim. The original PR for it made the Roc+ seem like a disc they were going to sell in Champ and Star, but other than some fundraiser Star's the only plastic I've seen it in is R-Pro.
Edit of the Edit: OK, so... They now have a Star Rancho Roc in Star along with the recreated Star San Marino Roc AND they have a Roc3 mold which they are selling in Champion. So now there are 5 Rocs: Rancho, Roc3, San Marino, Roc+ and Ontario.
My original assertion that Rocs were not all that confusing can be freely ignored.