The 3DS sounds like it's the DS2, basically. It's the second full-fledged DS system.
All DS games will play on any DS system—DS, Lite, DSi, XL. (Well, not ALL, since some, like Guitar Hero, require the GBA slot, which only the DS and Lite have.) DSi games (which I believe are only downloadable at this point) will not play on the DS or Lite, but will on everything else, including the 3DS. Or so it sounds.