Nearly everything is a system made off of subsystems. From our cells to our body to the family, city, country and planet we are living on.

Systems are the sum of interconnected parts, which act as a whole. However, systems are more than just the sum of their parts, due to synergies, which arise between their components.

A system can be defined by the entities/parts, which are within the system and outside the system.

Further, you can categorize them into natural- and man-made systems. The major difference is, that man-made systems are designed to archive a certain objective by the actions of their parts. In contrast, nature-made systems as a whole appear to not have a certain purpose. However, the behaviours of the system components can be interpreted as purposeful.