to start chances are you only have one actual physical drive in your computer but that is divided into 2 partitions which the computer treats as two separate drives
C is your system drive (normally) and is where windows itself is stored and works from
D is just more space! you can store your documents on it, install your programs onto it or whatever.
If your short of space on C then move some of your stuff like pics and ducuments onto D it makes no difference where they are stored as long as you can find them
You can also install most programs onto the D drive, when installing select custom and change the letter C at the begining of the install path to D and it will work the same.
So if your short on space on c move your pics, docs, music etc onto d... if your still short on space on C then uninstall some of the bigger programs and reinstall them on D (you can't just move the programs!)