When you are working on your computer, your work is in the computers electronic memory (memory is also called RAM). If the power goes off, the electronic memory is erased. This is bad - it means your work disappears. The memory is also erased when you turn your computer off, when you are finished working with it.
In order to keep your work safe while your computer is off, you have to put your work onto a disk. This is called "saving" your work. When you save your work, the computer copies your work from the electronic memory, into a file on a disk. So a disk is for keeping your work while the computer is turned off.
There are different kinds of disks: floppy disks, the hard disk (also called the hard disk drive, or the hard drive - the "hard disk" is inside the "hard drive"), CD's, etc.
The hard disk is the main disk in your computer. This is where all your programs are kept, and you can keep your data there, also.
When you are working (for example, when typing a letter in your word processing program), you should "save" your work every 5 to 10 minutes, in case the power goes off or in case something else goes wrong. This will copy your work into a file on a disk, where it will be safe from a power outage, and also after you turn your computer off.