There are generally seven elements of international relations theory: world perception, view change, conceptualization of power, definition of state's interest,state motivation, intensity of conflict, and does regime type matter (although this only matters for very few theories, most don't deal with this issue).
The main international realtions theories are:
Constructivism (although this isn't truely a theory)
They all view the answers to the elements of IR theory differently.
Realism: views the world as simple and easy to understand. Change is not real - because change cannot happen. Power is both a means and an end. State interests and preferences are based solely in terms of power. The intensity of the conflict is high, and regime types do not matter.
Neorealism (Waltz): views the world simply in terms of anarchy and polarity. Change can only happen in the polarity of the system because anarchy will never change (although if you read his theory, it doesn't sound as if change of any kind can happen). Power is a means, and is used to survive, which is the interest and motivation of states. Intensity is always high, and regime type doesn't matter.
Neorealism (Mearsheimer): views the world simply in terms of anarcy and balance of power. The world can change, but in order to change more than simply the balance of power, you need to change the anarchic nature of the world (and that won't happen). Power is a means, and there is latent (economic) power and effective (military) power. The state's interest is security, and all states will rationally act to maximize their security. Intensity is always high, and regime type doesn't matter.
Liberalism: The world is complex, and there are many factors that go into deciding how states will act. Change is real and can come about by the natural goodness of humanity and the autonomy of the individual. Power is a means to make other states democratic. The states' interests are defined through the political process becuase the government governs and acts with the consent of the governed. Motivations don't matter, because the natural goodness of man will prevail, even if unintentionall. The intensity of the conflict is low, and the regime types favor civil/political liberties and government by law with the consent of the governed (which means democracies and republics).
Neoliberal institutionalism: Views the world like realism, except that institutions can overcome fears about competition inherent in relaism. Change can only happen through insitutions, and the power must be in the institutions, otherwise they won't work. States concentrate on absolute gains and prospects for cooperations. By holding the state's interests constant, motivations can be said to be the same as the interests. The intensity is low, and regime types don't matter so long as states cooperate with institutions.
Constructivism: This is not a theory - it has no causal relations, it is simply an "analytical tool." The world view is complex, and based on the recipricol nature of interests and social norms. Change is real and comes fromchanging the identity and interests of the actors - which comes from changing ideas. Constructivism doesn't deal with power. Interests are defined by the process (Wendt) and develop from and with social norms. States are motivated by the intersts and identities of their citizens, which are accumulated to make a state identity and interest. The intensity of the conflict is low - otherwise realism works (Wendt). Regime types matter because constructivism assumes democracies - otherwise individual preferences cannot be accumulated to make state preferences.