DS, DS slim: That’s the first generation. 67MHz, 4MB RAM. Both consoles are identical apart from the form factor and play DS and GBA games.

DSi, DSi XL: Second generation, 133MHz, 16MB RAM, downloadable games, no GBA slot. GBA compatibility still exists via virtual console, but not with physical games. Adds a camera and integrated storage.

3DS, 2DS, 3DS XL: Third generation, 2x268MHz, 128MB RAM. The 3DS variants have autostereoscopic 3D displays. The 2DS is a hingle-less cheaper variant without 3D displays. Adds a left joystick. 3D cameras.

New 3DS, New 3DS XL, New 2DS XL: Fourth generation, 4x804MHz, 256MB RAM, second stick

So basically there are four generations. Each generation massively upgraded the performance while adding new features here and there.

Each of the generations (apart from the original DS) has size-variants that have bigger screens for a higher price and a larger size.

The DS and DS slim are identical apart from the fact that they managed to slim down the device with further development.