Social stereotypes are cases of metonymy, where a subcategory has a socially recognized status as standing for the category as a whole, usually for the purpose of making quick judgements about people. The housewife-mother subcategory, though unnamed, exists. It defines cultural expectations about what a mother is supposed to be. And because of this, it yields prototype effects. On the whole in our culture, housewife-mothers are taken as better examples of mothers than nonhousewife-mothers.
The characteristics of a given social stereotype may or may not have much basis in fact. Stereotypes can sometimes be a relatively value-neutral categorization of behavior (e.g. the view that most parents have a tendency to nag their children). On the other hand, when unjustified stereotypes are applied to groups, often the result is negative. Negative stereotyping is a key feature in prejudice, as racism, sexism, ageism, et cetera.
Advocates for public policy changes often have to overcome social stereotypes, while opponents of public policy changes often seek to reinforce them. For instance, in the field of civil rights, Martin Luther King, Jr. acted decisively against the social stereotypes of negative characteristics of African Americans, while his arch foe, FBI Director J. Edgar Hoover, sought to gather information to reinforce social stereotypes of African Americans.
Social stereotypes take the negative characteristics of some and apply it to all in a given, often arbitrary, category. They are a tool of propaganda and divisive politics.