It's for both. The ODD (Optical Disc Drive) adapter is used for powering the Xbox 360 DVD drive and also has a ModeB button for older Hitachi v46 , v47, and v59 DVD drives. It just connects to a normal molex 4-pin power cable in a normal desktop PC.
The hard drive adapter is for connecting the Xbox 360 hard drive to your PC without the need to take it out of it's enclosure.