There are many types of system, and the general definition is simply for when things work together. Examples of other systems include Physical Systems (such as weather systems, or the solar system), Biological Systems (such as the human body), Educational Systems (including schools and universities) and of course Information Systems.
These systems consist of 3 basic elements; Input, Processing and Output. Input are the things that enter into the system, for example foods may enter into a biological system like the human body, orbits and pressure systems are inputs to the solar system and weather systems. Processing is the transformation of inputs to outputs; this could be a chemical reaction, complex calculations, etc. Finally output is the result, where things leave the system moving on to other systems, for instance in the human body the food input is converted into energy and the output is movement.
For an Information System, data is the input and information is the output.
Systems become more useful if we add some ‘managerial’ functions, or a ‘feedback and control’ loop whereby data is collected about the performance of the system (feedback) and this feedback can be monitored to determine if the system is moving appropriately towards its goal, with adjustments being made where necessary.