Day by day, every of us fletches up in situations when it is necessary to decide upon a convenient "strategy" to attain the best possible outcome (from his point of view). Provided the result depends on herself only, possibly on some other factors that can be forecasted with some probability but that are independent of the decision of anyone else (as, for example, the fall of a rock from a hill where nobody stands), the situation is relatively clear. Once the decision depends on some other intelligent being, the situation is much less clear - here the GAME THEORY enters the scene.
In simple words, the subject matter of game theory is the analysis of an extraordinarily broad range of decision situations in which the outcome for a particular decision maker depends not only on his action, but on the action of one or more other participants, too. And it is not only about "games" in the sense we are used to imagine - chess, poker, bridge, baseball, etc. On the contrary, it is above all about conflicts of business companies, financial houses, troops, fighters, submarines, countries, nations, politicians, political parties, motorists, creditors, lovers of the same lady, males in rut, genes, ... In order to buiod a unified theory, all these and many other situations will be called "games" and their participants "players". Hence every "game" will be defined by its players, rules, the influence of their decisions on the result and by the preferences of particular outcomes by particular players.
On one hand, the aim of the mentioned analysis is the description of given conflict situations and understanding the behaviour of particular participants (it is mainly important in situations when the rules of the "game" are too complicated and there are too many players), on the other hand - and it is much more interesting for praxis - it provides the players the instructions which strategy they should adopt (it is not always possible to determine the "best" strategy; nevertheless, the game theory can at least give an advice which strategy is "better").