Sometimes you will run into a game download that will have an ECM file.
For instance StreetFighter.bin.ecm.
Well you wont be able to burn this to a disc and play this game.
Drag and drop StreetFighter.bin.ecm onto the UnECM program, and it will extract to a StreetFighter.bin file. Then you are ready to burn.
ECM is another compression method. Can compress more by first using ECM, then using winrar/7zip, thus making the file you download smaller.