Hard Drive - Hard Disk or HDD is the same thing, it is a physical disk that we use to store all the smallest files on our computer.

The difference between hard disk and RAM is that the hard disk is the thing which comes in the store, but RAM comes in handy to run the things stored in that storage.
When we turn off the computer, anything that is found in the RAM gets cleared. But when the computer is off in HDD, the data is not erased.

Most hard disks are 5400 rpm or 7200 amp, obviously, 7200 RMP's hard disk is faster than 5400 rmp.
Whenever we store the data on the Hard Disk, it is stored in a piece, and many times those pieces go into different parts of the Hard Disk. When there is a lot of data, on our hard disk, our computer has a little difficulty in finding things on the hard disk, which makes it slow down.
Whenever you save any file on your computer, that file is saved on your computer's hard drive, if you have interest in parts of the computer then you should definitely read about the hard drive.