A system consists of inputs, components and outputs. Inputs involve materials or information entering the system which are then changed by the elements within the system (the components). The components that make up the overall system can be described as sub-systems. The output from a system is made up of the items or pieces of information that leave the system.

Within an Information System, there are people identified by role or position as follows:

• beneficial users
• immediate users
• related workers

There is a boundary which separates the contained system from its environment. Inputs and outputs come from and are put into this environment surrounding the system. The system has an organised structure within its boundaries where relationships and interactions occur between the different parts of the system. Good systems always have a purpose or well defined goal. An Open System exchanges input and output of some form with its environment whereas a Closed System does not interact with its environment.

An Information System can be defined as a collection of programs running on computers which interact with each other as well as humans to provide the necessary information needed to make decisions within an organisation. Nested systems are where there are several internal systems that link to each other, each being made up of multiple components.

"Cohesion" describes how well each sub-system within a system is designed around an individual task or function of the system. High (or "good") cohesion therefore means that the sub-system deals with a single function of the overall system and ONLY that function. This concept also applies to good object-orientated program design (e.g. Java).

"Coupling" refers to how independent each sub-system is in relation to other sub-systems. Low (or "good") coupling means that a certain sub-system has limited interaction with other sub-systems and therefore is unaffected by changes that are carried out within other sub-systems.

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