For our purposes a system is a set of objects interacting in accordance with certain laws of nature. There is a degree of arbitrariness in defining a system. We get to choose what to consider to be the system and everything left over is the system's environment. If our system behaves in unexpected ways the first place to look for problems is in the system definition. Perhaps we have not included all the pertinent pieces in our system definition.

At any instant in time a system is characterized by a set of numbers called its state. For very simple, well behaved systems like a clock pendulum for example, if we have a decent approximate knowledge of the state of the system at any instant, we can calculate a good approximation of its state at any future time.

For complicated systems involving many moving parts, or simple systems behaving badly (chaotic systems), calculating a future state from a current one becomes too hard. All we can do is use the second law of thermodynamics, or chaos theory, to make some educated guesses about the system's future state.