pepsi, the disk is only the device where the backups are being stored. It works the same like your disk in your PC. But you still need a program that will transfer your files to the disk - or you can also copy and paste files/folders to that disk. But that would not be very practical if there are a lot of files/folders.
Some disks come with backup software. But that is often not that great. We recommend free Macrium with which you can backup (image) whole partitions. It is very much recommended to backup your C partition so that you can always restore your system when it goes on the blink.
Here is a Macrium tutorial to get you started. If you have any questions, just ask. Imaging with free Macrium