The Nintendo DS (NDS/DS) is a video game handheld console made by Nintendo which was available from 2004 to 2011. It uses two 32-bit ARM CPUs (a main processor and a coprocessor). A revision came out later, the Nintendo DSi. MelonDS can handle ROMs for both of them.
MelonDS is a NDS and NDSi emulator that is fairly new but has less issues that older emulators of its kind. NDSi is a work in progress, however.