== What is a computer? ==
You have probably had a lot of experience with computers. They are a type of machine or device which can be programmed. Computers range from the desktop or laptop you are probably reading this on, to a computer in a vending machine or supermarket checkout. Computers are everywhere in out modern life, and are incredibly important to the functioning of the modern world.
All computer systems work in a similar way. They have inputs, processes and outputs. Take making a phone call on a mobile phone as an example.
== Reliability of computer systems ==
Considering that computers are currently used for so much in the modern world, keeping them running well and constantly is an important consideration. A Crittical system is one which needs to constantly be running, or else there is a risk of death, serious injury, or financial problems.
Other examples of critical systems include utilities, banking, emergency communications and air traffic control.