De facto segregation refers to racial, status or class segregation that arises between groups politically or socioeconomically, done through fact and not through formal legislation.
An example of de facto segregation would be the social apartheid in South Africa, in which white people were regarded as superior, while people from other races were discriminated and segregated.
On the other hand, de jure segregation refers to the segregation that is done through a legislation. An example would be the Jim Crow laws in United States during the 19th and 20th centuries.