I want a Nintendo handheld console, but I don't know which of those suits my needs.
What does the 3DS have that the DSi doesn't?
What does the DSi have that the DS doesn't?
Also, can a DS play newly released games? Or do they require a 3DS?
The 3DS has a 3D feature that does not require 3D glasses. Just bare eyes. It also has an analog stick and a home button. Plus a stylus that you can extend and augmented reality where you put down a card and things pop out.
The DSi has the feature of DSi Ware where you can get games for a cheap price without leaving your couch. It also has a camera to take pictures with, and a built-in Internet browser.
The normal DS is just a basic insert a cartridge and play.
A DS cannot play 3DS games, nor can a DSi.
DS is, well, your basic DS. It can play DS games... and that's about it. Oh, it also plays GBA games. The other systems can't.
DSi is DS + internet browser + camera. It also has downloadable DSi games (almost all require purchase of DSi points).
3DS is DSi + 3D capabilities. It also has a lot more downloadable games, including almost all DSi games, and some included games and videos for free. In addition to that, it plays both 3DS and DS games. And it has some other features like tilt sensors.
New DS games are still coming out, but the releases are getting less and less. 3DS games require a 3DS, and as time goes on, most likely DS games will stop being released in favor of 3DS games.