EMP - eMusic Music Download File is a music file that's used by eMusic Download Manager. This format is really great because eMusic Download Manager allows you to download .MP3 files and play them in other music programs like Apple iTunes and Windows Media Player. This files are very versatile and really take the hassle out of converting audio files.
The only downside to this format is that the eMusic Download Manager is not compatible with EMP files. It is also important to note that these files by themselves do not contain any audio data, they are only used to convert existing files.
Overall, these files are a great way to play music and media across different platforms and applications.