Apple has model numbers for all their products. The original iPhone was iPhone 1,1. The iPhone 3G was iPhone 1,2. The current iPhone 4S is iPhone 4,1. The next iPhone will be iPhone 5,1. Likewise, the original iPad was iPad 1,1. The iPad 2 was iPad 2,1. And the new iPad is iPad 3,1. The Retina MacBook Pro is MacBookPro 10,1. Following pre-production code names, that's how Apple classifies things internally.
Within models, there are variants. The iPhone 3G was a radio variant of the original iPhone 1,1 but was fairly similar otherwise, hence iPhone 1,2. The iPhone 3GS got a whole new chipset, so also got a model bump to 2,1. Likewise, the iPad 2 was a leap ahead of the original, so it got iPad 2,1. At least the Wi-Fi version did. The GSM/AT&T version got iPad 2,2 and the CDMA/Verizon version got iPad 2,3. When Apple updated the internals again this spring for the newer, lower cost version, that variant was iPad 2,4.
iPad 2,5 has been showing up in developer logs for months and iPad 2,6 was noted yesterday by Marco Arment on Marco.org. So what are they?