Yes there is a different slot for them.
You can play 3DS, DSi or DS games. (Note: DS and DSi games do not display 3D visuals.
No
No, dsi games work in a 3ds, wont appear 3d though, but the dsi does not have the ability to play 3ds games.
Yes
The 3DS can play the DSI and regular ds games and Yes Of course the 3DS can Play the 3d games but other nintendos can not play the 3D games.
No, the DS and DSi games will not be in 3D.
There are a few 3D games for the DSi. Legend of Kay is one, though I wouldn't actually recommend it.
A faster processor and more RAM was added to the DSi. This will allow for better looking and smoother gameplay.
You can play in 3D on a 3ds and a Dsi cant.
Yes, the 2DS can play 3DS games but just without the 3D.
no you can't play dsi games in ds but you can play ds games in dsi.
You can import most DSiStore games onto a 3DS from the 3DS' memory management menu. You can also play Monster Finder, which is the only true 'DSi game'.