The four basic functions of a computer system are as follows:
Let's look at each individually:
Transferring of information into the system. This may be through a user input device - i.e. keyboard, mouse, scanner etc.. Or though previously loaded software/program, cd etc.
Output is the exact opposite of input. Output is the function that allows a computer to display information, from the system, for the user. This can be accomplished through the monitor (or other graphical display), printer, speakers etc.
This is where the computer actually does the 'work' - manipulating and controlling data over the entire system.
Most computers are able to store data both temporarily (in order to process), but also long-term (i.e., permanently). Storage takes place on hard-drives or external storage devices.