Revision A and B were, obviously, the first versions and are very similar. Revision C is a little faster and has the LED lit screen (although I think the very first ones didn't). The presence of the star button on the top right hand button rather than the previous symbol was the easy way to differentiate these newer units at the time.
They stayed at Revision C for quite a while but eventually updated to Revision D. I think this is when the newer, bigger, faster hard drive was added. Since then the revisions have increased rapidly. It seems like this is as they have upgraded the radio to be compatible with the latest vehicles however the actually capabilities of the units haven't really changed. The most recent versions now include DAB which is probably the most significant upgrade of recent. I think this was introduced in Revision H, but I'm not certain on that.
All the recent firmware updates seem to be "officially" compatible back to Revision D however I am successfully running the 4120 firmware on my Revision C unit. This firmware version introduced Radio Text which has long been a missing feature from the RNS510 for unknown reasons. The latest firmware in now in the 5000s but this seems not to be compatible with anything but the latest units.
Maps are currently at Version 9. You can purchase the DVD off your VW dealer or if you don't want to pay then they can be "obtained" off the Internet and burn your own DVD, but we don't condon illegal activity here!
To install new firmware or maps you just stick your update DVD into the unit and leave it to do its thing for an hour or so until its finished.