Stereotypes typically rely on salient features based on experience with group members or shared by those in one's culture. Stereotypes usually ignore the variety of features true of people in a group.
Stereotypes are usually negative but positive stereotypes exist. It is very difficult to change a stereotype.
Group stereotypes are those cognitions that consider all members of a recognized group to have the same characteristics. Commonly recognized groups can include religions (e.g., Christians, Jews, Muslims), political parties (e.g., Republicans, Democrats), organizations (e.g., Red Cross, ACLU), businesses, and nations (e.g., Americans, Germans), Race (e.g., Blacks, Whites), Ethnicity (e.g., Mexican American, Native American).
People may hold separate stereotypes about the people in various subgroups (e.g., Catholics, Methodists).
Socially constructed category stereotypes
Socially constructed category stereotypes are stereotypes that people apply to categories of people, which may be based on one or a few features. Social categories often include demographic characteristics like age categories (seniors, adolescents), gender categories (women, men, gays, lesbians), ethnic categories (e.g, Whites, Blacks, Caucasians, Native Americans), social values (liberals, conservatives).
Research on stereotypes
See the 2000 edition of The New Fontana Dictionary of Modern Thought