Before we get into anything else we need a basic understanding of how a computer stores data. Inside the computer box there are many different pieces of hardware used for storing data. One of these is the memory. A Computer’s memory is where it stores the data that is currently in use. So, for example, when you have a letter open and are looking at or working on it the computer stores it in memory. All the software currently running is stored in memory to. This kind of memory is called RAM (Random-Access Memory). Random-access means that any part of the information may be read or changed randomly, the computer does not have to look through all the data in order so that it can find the right piece. There is another kind of memory called ROM (Read-Only Memory). Read-only means that the data on it cannot be changed. ROM is used to store basic information that every computer needs.

Random-access memory RAM is used to store the data the computer is currently using.

Read-only memory ROM is used to store the basic information that every computer needs.

When a computer is turned off, everything in RAM is erased. Because this would cause you to lose your work all the time, disks were created. Disks are round flat objects, but in computer slang they are pieces of data storage hardware that do not need electricity to keep the data stored. There are other things that do this that are not disks, however the things inside of all things called disks are round and flat, so that is how the term came to be. All disks need to be in another piece of hardware that acts as an interface between them and the rest of the computer called a drive.
  Figure 4.2 - a floppy drive