playing roms on the ds

hey i was wandering  a while back i  heard about this thing where you can download a game into a mini sd card and plug it into this special  ds cartridge that plays games off sd cards on the ds. i forget its name i think its 3rd party though anyway would it be able to play roms of gba games or nintendo games otherwise unavailable in canada on my ds that i download on my computer for a more authentic experience or would my ds not of the software  to run em.

i dont endorse piracy this is just general curiosity over a legal product