I would just like to add that black segregation was already much prevalent when the Civil Rights movement began; that's why there were "designated" seats for blacks in buses. This was the result of Jim Crow laws, a series of laws at the state and local levels that allowed the segregation of blacks and whites in public areas of life. The official government policy for segregation was struck down in 1954 but society didn't follow. I think that the Rosa Parks incident was sort of the tipping point that started the grassroots movements that we refer to as the Civil Rights Movement. This was not the only incident at the time. The movement itself was the result of years of systematic segregation and discrimination against blacks.
The Civil Rights movement began on December 1, 1955, when Rosa Parks (1913– ), a black seamstress, refused to cooperate with a segregation law. As she boarded a public bus in Montgomery, Alabama, she took a seat in the designated "black" rows in the back. When the bus filled up she was asked to move so that a white man could have her spot. She refused to give the man her seat and was then arrested. This event sparked what would become a national movement of resistance to racial segregation (separation of black people from white people) and discrimination. Local black leaders organized around Parks with Baptist minister Martin Luther King Jr. (1929–1968) as their leader. They decided to start a citywide boycott of the Montgomery bus system on December 5, 1955. The boycott lasted 382 days and ended only when the case had reached the Supreme Court, which ruled that segregation on buses was unconstitutional (against the law). This marked the first momentous victory of the Civil Rights movement.
Further Information: Ballard, Sara. Free at Last: A History of the Civil Rights Movement and Those who Died in the Struggle. New York: Oxford University Press, 1994; Parks, Rosa. Rosa Parks: My Story. New York: Penguin Putnam Books for Young Readers, 1991; 4CivilRights.com. [Online] Available http://www.4civilrights.com/,4Civilrights, November 1, 2000.