Along with the World Bank, the IMF was founded on the day the Bretton Woods Agreements were signed. Its first mission was to support the new system of standard exchange rates.
When the Bretton Wood fixed rates system came to an end in 1971, the main function of the IMF became that of being both policeman and fireman for global capital : it acts as policeman when it enforces its Structural Adjustment Policies and as fireman when it steps in to help out governments in risk of defaulting on debt repayments.
As for the World Bank, a weighted voting system operates : depending on the amount paid as contribution by each member state. 85% of the votes is required to modify the IMF Charter (which means that the USA with 17,68% % of the votes has a de facto veto on any change).
The institution is dominated by five countries : the United States (16,74%), Japan (6,23%), Germany (5,81%), France (4,29%) and the UK (4,29%). The other 183 member countries are divided into groups led by one country. The most important one (6,57% of the votes) is led by Belgium. The least important group of countries (1,55% of the votes) is led by Gabon and brings together African countries.