The 18 and 20 have the same voltage, but different battery design and a little smoke and mirror talk. When they came out with the new Lithium Ion platform, they chose to advertise them as 20V in lieu of 18V. The older NiCad platform was advertised as 18V (a nominal voltage) while hyping the newer Lithium Ion at 20V (Maximum voltage). The engineers told us the new platform was going by "20V" to avoid confusion between the NiCad and LI batteries. I think most of us at that meeting thought "BS - they want the new platform to sound bigger and better".
Yes, there are adapters that allow 20V batteries to be plugged into 18V tools. In a lot of cases it works, but some tools don't have sufficient space or clearances. Examples are my little 120/18V vacuum. The battery cover can't be closed. I haven't tried it on my trim saw, but a bud said his trim saw won't cut to full depth because the (adapted) battery bumps the work piece.
DeWalt engineers told us the 18V NiCad will still be around for a long time - or at least until they come up with a workaround for batteries used where they are exposed to radioactivity. The LI cells have trouble there.
When the 18V tools came out I thought there wouldn't be much room for improvement. I was wrong about that. In 18V I still have a bunch of tools, but have added several 20V tools. The LI batteries run longer, especially with brushless tools and you can do more work with them. My 20V grinder runs a whole lot longer than my 18V did. In truth, the 18V grinder was called a "Cutoff Tool" by DeWalt because they realized battery life limited the use. (We bought a few 60V grinders at work and they rival corded models) I recently bought a 7-1/4" 20V circular saw. I've only used it a couple times, but it feels like a real saw and has the blade to the right side - my preference.
