Social structures are how societies are organized. One common example of a social structure is a patriarchy. In this kind of society, prevalent through much of the world, society is structured according to a binary notion of gender, with men on the top and women on the bottom. In the more extreme manifestations of this system, men own the vast bulk of the property, and with marriage, the woman is understood to "merge" with the man so that any property she owns becomes his. Women do not vote in this system of social organization, the children in a marriage "belong" to the man and take his last name, and men hold all the positions of power and authority in the society. While this can seem cruel and unjust—and it often was—proponents saw it as the only way to insure social stability.
In the southern plantation social structure that existed in the American South prior to the Civil War, one's place in the social structure was determined by birth, with men at the top, white women below them, then black people below white people. It was believed that, ideally, if everyone simply smiled and graciously accepted their predetermined slot in the social order, the society would function smoothly. Of course, as in patriarchy (and this society, too, was patriarchal), those "born to" the classes below tended to rebel against their lack of privilege.
An alternative social structure, one many countries have gravitated towards in the last one hundred years, is egalitarian: all citizens of a nation, regardless of gender or race, are treated equally under the law and are supposed to have the same opportunities to rise in the social order.
In communist countries in the twentieth century, all hierarchy was supposed to be flattened into a classless society. People referred to each other as comrade, and women and minorities were supposed to be treated equally. In the Soviet Union, women served beside men in the army and worked beside men in the factories.
It is difficult to achieve purity or equity no matter what the social structure. In highly hierarchical structures, people tend to get out of their assigned "place," and in more equalitarian structures, hierarchies have a way of popping up.