Judging by the size of the D: drive, and its name, "DATA", that is probably exactly what it is intended for; storing your music, pictures and other files.
Some laptops have a really small D: drive, maybe named "RECOVERY", which contains files needed to restore the laptop back to factory settings in the event of a problem. You shouldn't store data on these (and often it will try to stop you).
It's probably not actually a separate hard drive; it's just part of the same hard drive as the C: drive, called a "partition".
Although you can copy your files into the D: drive, you might also want to set your My Documents folder to be on the D: drive too. Here is a page with instructions for moving your My Documents folder: http://www.makeuseof.com/tag/how-to-move-the-my-documents-folder-and-save-your-data-windows/
One advantage of storing your data on a separate "drive" to the operating system is that you can, in theory, reinstall the operating system without wiping away all your files. It can also potentially bring speed increases.
You should keep all your programs and operating system files on the C: drive.