attrocities

[Suicide Junkie]

Load them with SE4.

Basically, they are saved race settings.
On the game setup screen, click the "add existing race" button, and select the EMP file.

Handy if you use the same race settings all the time, and used when players need to send their data in to a multiplayer game host without revealing their secret settings.

[Atrocities]

EMP is short for Empire. So EMP file is really just a saved Empire File as Suicide Junkie has stated.