Flash cart. Look it up.

>>185
Any other info you can give me? A brand name or site to look or something? How do you use them?

>>186
http://en.wikipedia.org/wiki/Nintendo_DS_storage_devices
You're going to need a second gen flash cart to play HGSS, and depending on which kind you get, you'll probably need an microSD/SDHC card. Some of them come with USB support, in which case you obviously wouldn't need an SD card.
It all depends on what you want to use, though I myself have never purchased one (I'm a poorfag), so I can't quite tell you which are the 'good brands' and which are the 'bad'.

Cyclo DS was the one to buy, just after I got my R4 (don't get an R4). But that was a while ago.
There was another one which also updated it's firmware and had continued support by the creators.

Research a lot before buying. I think there are better communities to ask about flashcarts.
Also, if you have a DSi, you need certain flashcarts for that.

If you're getting a DSi for use with a flashcart, don't. The DSi is pretty much the biggest pain in the ass to run flashcarts on EVAR.

For the original DS or DS Lite, your best bet would probably be an Acekard2, since the AKAIO custom firmware for it is constantly being worked on, and it gives you built-in support for the EZ 3-in-1 (which is also a good accessory to get). You'll need a transflash/microSD card to store games/saves/etc on, so all in all you're looking at maybe 25-30 dollars after tax and shipping if you shop smart. The 3-in-1 will probably be another 10 to 15 depending on where and when you get it.