A system is a set of things - people, cells, molecules or whatever - interconnected in such a way that they produce their own behaviour over time.
The system, to a large extent, causes its own behaviour! An outside event may unleash that behaviour, but the same outside event applied to a different system is likely to produce a different result.
A system is an interconnected set of elements that is coherently organised in a way that achieves something.
Many of the interconnections in systems operate through the flow of information. Information hold systems together and plays a great role in determining how they operate.
A system generally goes on being itself, changing only slowly if at all, even with the complete substitutions of its elements - as long as its interconnections and purposes remain intact.
Stocks generally change slowly, even when the flows into or out of them change suddenly. Therefore, stocks act as delays or buffers or shock absorbers in systems.
A feedback loop is formed when changes in a stock affect the flows into or out of the same stock.
Balancing feedback loops are equilibrating or goal-seeking structures in systems and both sources of stability and sources of resistance to change.
Reinforcing feedback loops are self-enhancing, leading to exponential growth or to runaway collapses over time.
The information delivered by a feedback loop can only affect future behavior, it can't deliver a signal fast enough to correct behavior that drove the current feedback loop.
One of the central insights of systems theory, as central as the observation that systems largely cause their own behavior, is that systems with similar feedback structures produce similar dynamic behaviours, even if the outward appearance of these systems is completely dissimilar.
In physical, exponentially growing systems, there must be at least one reinforcing loop driving the growth and at least one balancing loop constraining the growth, because no physical system can grow forever in a finite environment.
Loss of resilience can come as a surprise, because the system usually is paying much more attention to its play than to its playing space. One day it does something it has done a hundred times before and crashes.