A RAR file is a kind of data archiving file type used for compressing large or multiple files into a single container for easy download. Second only to ZIP files in popularity, the RAR file type takes longer to compress than ZIP, but offers a marginally better compression rate. An NDS is a ROM of a Nintendo DS game intended for use with a Nintendo DS emulator. Properly used, an emulator and an NDS file will allow you to play your favourite Nintendo DS games on your computer.
Download your RAR file containing your NDS ROMs.
Download WinRAR (see Resources).
Install WinRAR using its autorun feature.
Double click your RAR file. WinRAR should automatically uncompress your RAR file, giving you access to the NDS files inside.