About the double topic creation - it happens. I didn't see anybody fly off the handle about it, but maybe they did in the other one. But like I said, internet acts up, you click the button, it happens to all of us.
I'd suggest looking at the links Rydian posted. They're great ones to look at if you want to learn more about a flashcarts and what they do and how they work.
But simply put, it's a custom cartridge (which in itself is legal, but not licensed by NIntendo) which has a slot for a microSD card, which you can put homebrew, music/media, and
legally backed up games (but of course people pirate them, which is illegal. Keep in mind, asking where to get roms on gbatemp is
very against the rules. If you choose to want to find them, they are a very simple google search away, but on gbatemp, you will not find them nor where/how to get them). But you essentially take your games, homebrew, and media and put it on the SD card and play them through the console; what that multi-cart does that you linked, except that would be illegal because you're not buying legitimate copies of the game (likelihood of getting caught is even less than downloading roms though). Problem is, Nintendo pushes out system updates all the time to try to block flashcarts; a big one just came out that
may permanently block all carts except the Supercard DSTWO and one of the R4 3DS carts (don't remember which). Anyway, the manufacturers of these carts put out their own updates for the carts to get them running, despite Nintendo blocking them. However, that multi-cart isn't updateable. Once a 3DS update comes out, that will only be usable on (possibly) a DSi, and if not, definitely a DS/DS Lite.
The Supercard DSTWO is what I have. I think it's a great cart, as do many others. Setting it up is a tiny bit complicated (but much easier if you read the directions on their site), but once you do, you would just need to obtain roms somehow, and put them on your SD card and you're pretty much good to play. You can optionally run them through a program called NDSTokyoTrim, which just gets rid of the useless garbage data (padding) in the rom. Basically, a game could be written to a 32MB flash chip on a retail cart, and the rom would be the same size, but the actual game itself could be something like 21MB, meaning there's 11MB of padding in there to make it fit, and you don't need it. But if you have a big enough microSD it's seldom an issue. Back when I had a 2GB microSD, it was. With my 8 or my 16, it's not at all.
Anyway, you could absolutely take the moral high ground and choose to not get one, and just go legit with it. But should you choose to look into it, you've got plenty of resources on this site to look at.
Cheers