There are various categories under which storage devices can be classified;

1. The duration they store information;
2. Mobility and;
3. How they function.

Storage devices in this category can either be;

1. Secondary storage devices
2. Primary storage devices

Storage devices can also be categorized according to mobility. I.e.

1. Fixed storage devices and;
2. Portable storage devices

Storage devices functions in different ways depending on the technology used to manufacture each one of them. They include;

1. ​Solid state storage device
2. Magnetic storage device
3. optical storage devices

This is the ability for a computer to accept data either through input devices such as scanners, keyboards, mice and others or through information retrieved from storage devices.
Input of data is a main function because all other functions depend on the incoming data for processing and output.

Processing and storage are sometimes broken down as separate basic functions of a computer. However, looking at this critical idea keenly, we notice that processing  and storage are sometimes inseparable this is because processing devices rely heavily on storage devices vis a vis.
Processing is an act where a computer converts raw meaningless piece of information called data into a more meaningful piece of information.
This is normally done with a closer help of primary storage devices such as the RAM, and special types of memories such as the registers, buffers and cache memory.
​Processing is a main function of a computer because of the conversion, manipulation of numeric data and logical decisions.

This is an act for a computer to display the result of a processing activity. This can be done using output devices such as the monitor, printer, plotter, speakers etc.
The functions of a computer system cannot be complete without the output, simply because it gives a feedback of the result for the computer user to digest, change, amend, store or print.