DS is a the big silver one, has GBA backwards compatibility.
DS Lite is smaller with brighter screens and slightly improved battery life and a new D-pad design.
DS Lite XL is a larger version of the DS Lite.
DSi has brighter screens, the addition of a camera, the GBA backwards compatibility removed and a Media card slot for storing games on downloaded from the the DSi online service.
3DS is a new handheld, not a DS, it is about as powerful as a Wii, it has a touch screen and a 16:9 3D upper screen that shows 3D via depth rather then leaping out at you (like them cardboard theater things, different stages inside the screen) it will have some form of video service for watching 3D movies, take a media card (proprietary format) for downloadable content, have backwards compatibility for DS, an internal gyroscope and 2 stereoscopic cameras as well as 1 internal camera.
it will also have region locking in someway that has not been explained and costs far too much money (more then 200 quids).