Systems are interactive and coordinated sets of entities that are capable of consistent behaviour. For some systems, like the solar system, the consistent behavioural outcome is a stable and predictable dynamic equilibrium of the elements. Other systems, like gear-boxes, transport, lighting or computers, although needing stable and predictable internal interactions to be able to operate effectively, have a clearly identifiable output... in the form of torque, delivered passengers, lumens or computations. Systems are invariably complex, and it can be a major difficulty to decide how the elements may be conceptually 'simplified' in order to determine or understand its behaviour. For 'natural' systems, determining which simplifications interact significantly with one another will determine the merits or otherwise of any conceptual models of those systems that are proposed. The solar system, for example, can be considered as a complex gravitationally interactive entity in which the sun and the planets are conveniently simplified to be objects possessing mass which move in time and space. This conceptual system-model thereby assumes that all the other (chaotic) aspects of reality, like solar wind, geologies, atmospheres, chemistry, life, etc do not influence the functioning of the system. As systems become more complicated, so is it increasingly difficult to determine the influence of the elements. The cycle of global warming and cooling that the earth climate system experiences has many probable sources of influence. Solar sun-spot cycles, earth orbital cycles, galactic cycles or even cosmic radiation may all be significant extra-terrestrial drivers, and of course atmospheric composition variations, continental relocations, volcanic activity and biological influences may all be terrestrial influences. Contrived systems, like those of engineering, agriculture or finance, are goal driven, but although the object of the construction of the system is to achieve a predetermined outcome, this is rarely attained by the first prototype. In fact all such human contrived systems... like aircraft, food production or the economy... are being continuously modified to improve performance and cope with changing circumstances.
The defining characteristic of a system is that the elements are coordinated and the output is consistent. The coordination of the elements of a system is achieved in many ways. Thus for example, mechanical systems are coordinated by gears and pulleys and chains and cables... computers by clock pulses, binary instructions and interrupt signals... biological systems by chemical messengers and electrical pulses... social systems by rules and hierarchical bureaucratic messages...and so on... Basically, any system of communication whatever... existing or yet to be devised... could be utilized to link a set of elements, influence their behaviour, and thus potentially create a system.
Nothing is free. Systems are not without cost any more than lunches are. All systems use energy in order to function, and many are designed to deliver energy in the form of resources as an output effect. All the physiological systems of living things... respiration, circulation, digestion etc... have their energy demands which must be provided by food or fuel reserves. All human social systems... education, economic, military and so on... have a socio-economic cost, and much of the endless restructuring of these systems revolve around efforts to try and make them more 'efficient'. Even seemingly 'intellectual' systems like the numerical system or a classification system still need purely cerebral or computerized energetic input in order to obtain an outcome.
Some systems are cyclic and capable of repeatedly carrying out their specific behaviour or function with a minimum of energetic input. Digestive, timekeeping and monetary systems usually manage to carry out a number of repetitions before they fail due to fatigue or poor design. Some on the other hand are one-shot systems, because vital elements are consumed in the process of achieving the aim. Candles, disposable nappies and poisonous defensive barbs destroy their resources in the process of carrying out their design goals.
Many systems must be able to adjust their capacity to cope with the level of demand. The circulation system needs to respond to the demand for increased exertion. An education system needs to be able to manage resources as student populations vary. Predicting demand is often a difficult exercise, so in order to cope, such systems would normally keep resources in reserve so that any unexpected increased in demand can be at least partially accommodated.
Some continuous dynamic systems alter their state and/or output in response to levels and changes in that output... a process called feedback. If the input is decreased when the output increases... or vice versa... (known as negative feedback) prudent engineering design can ensure system stability. Electric generation systems and control systems in general all rely on negative feedback to achieve satisfactory functioning. They can maintain operation within acceptable limits of a chosen output set-point. If however the input is decreased when the output decreases... or vice versa... (known as positive feedback) then the system will head towards the behavioural extremes of non-functioning on the one hand, or explosive disintegration on the other. One suspects that monetary systems fuelled by greed are not devoid of the influence of positive feedback, and that a more effective influence of negative feedback would not go amiss.
Systems can be themselves elements of other systems and participate in a hierarchy of systems. Thus the braking and electrical system of a car are subsystems of a vehicular movement system, which is in turn an element in a roading transport system, which is in turn and element in a food distribution system... and so on. All the physiological systems of digestion, respiration, circulation, etc are subsystems of the main living entity system which is dedicated to ensuring genetic material is deposited in the appropriate locations. It is probably reasonable to think of the entire universe as being an infinitely complex system of hierarchical systems, but whether that is at all helpful or not is another matter.
Unless the effects of an aggressive and chaotic universe are compensated for, no system can endure without failure. To continue operating, all systems... be they physical, social, or conceptual... need a regular and comprehensive program of maintenance. Easy enough for a water reticulation system, but not so easy for a solar system...