Segregation technically refers to any system of enforced separation between groups of people. In the United States, it usually refers to the system set up (de jure in the South, de facto in the North) that required African Americans and European Americans to stay separate as much as possible. They had to live, work, shop, eat, and drink separately, and even sit in different sections of public transit systems.
In the South, this was enforced by lynch mobs and the force of law. African Americans were not allowed to attend the same schools as European Americans, and were
sometimes always beaten or hanged for trying. In the North, the system was similar, but instead rested on the civil sphere to enforce it. For example, most banks and real estate companies would not consider helping an African American family to move into a "white" neighborhood. This then resulted in separate schools for the "races".
The system was, for the most part, destroyed during the civil rights movement. However, many cities in the US are still somewhat segregated, although this is blamed on tradition.
 South Africa
Under apartheid, South Africa was ruled by a racist caucasian minority (blame the Dutch?). Worldwide pressure from college students broke this horrible system in the late 1990s via divestment pressures.