It may be a seperate drive or just another partition on the same drive as the C partition is located at. Anyhow, you can use it for anything you would use C or any drive for. When installing programs, you can select to install it to D if there isn't enough free space on C. When you save a word document, picture, video file,...etc, you can select to save it to D. It just depends on how you want to organize files. If C is an SSD instead of an HDD, you will want to install programs that you need faster access to on C as SSD's are much faster than HDD's.