Relations among states take place in the absence of a world government. For realists, this means that the international system is anarchical. International relations are best understood by focusing on the distribution of power among states. Despite their formal legal equality, the uneven distribution of power means that the arena of international relations is a form of ‘power politics’. Power is hard to measure; its distribution among states changes over time and there is no consensus among states about how it should be distributed. International relations is therefore a realm of necessity (states must seek power to survive in a competitive environment) and continuity over time. When realists contemplate change in the international system, they focus on changes in the balance of power among states, and tend to discount the possibility of fundamental change in the dynamics of the system itself.
The following key thinkers all subscribe to these basic assumptions in their explorations of the following questions:
(1) What are the main sources of stability and instability in the international system?
(2) What is the actual and preferred balance of power among states?
(3) How should the great powers behave toward one another and toward weaker states?
(4) What are the sources and dynamics of contemporary changes in the balance of power?
Despite some shared assumptions about the nature of international relations, realists are not all of one voice in answering these questions, and it would be wrong to believe that shared assumptions lead to similar conclusions among them. In fact, there is sharp disagreement over the relative merits of particular balances of power (unipolarity, bipolarity and multipolarity). There is also much debate over the causal relationship between states and the international pressures upon them, and the relative importance of different kinds of power in contemporary international relations.
see details: REALISM – http://www.slideshare.net/mcrosiqin/realism-44724016
and read, Fifty Key Thinkers in International Relations – xa.yimg.com/kq/groups/23106968/639078494/name/0415162270.pdf