From the Apple menu, About this MAC, More Info you can find your Model Identifier: e.g. MacBookAir2,1
RevA would be 1, rev B 2, and RevC2,1. Each rev had a basic version with slightly slower processor and HDD and a slightly faster processor and an SSD (although special orders could confuse that issue).
RevB was a major upgrade going from a custom version of an older processor to a mainstream version of a newer higher density lower power processor, going to much more powerful graphics, going from a PATA to a SATA drive interface etc. Major rework of the internals, completely new motherboard.
RevC was a much more minor upgrade - with the only readily apparent change being a processor speed bump. You got a newer generation of SSD and a different screen as well but no major redesign.
Who knows what the RevD will bring and when.
That's not true. Since October 2008, the model has been 2,1. What people here have called a rev C is still the same rev 2,1 that has had an extremely minor CPU bump not a revision to the MBA. The main basis of the machine has stayed the same since October 2008.
So January 2008 was rev 1,1. October 2008 was rev 2,1. When Apple bumped the CPU the next time, it stayed rev 2,1. The only reason there was a bump is Intel bumped the SL9x00 series chips. So Apple probably got the 2.13 for the same price as 1.86 was before.
I expect the next MBA to be a new form factor and a new chipset with a real rev C designation to 3,1.