1. Stereotype is a preconceived image of a particular person, thing or situation. They are used primarily to define and limit people or groups of people in society. The Stereotype is also widely used in Humor as a manifestation of racism, xenophobia, machismo, misandria, religious intolerance and homophobia.

2. Types of stereotypes
Economic and social stereotypes.
Beauty stereotype.
Gender stereotype.
Ethnic, racial and cultural stereotypes.

3. Yes, it's a problem because you shouldn't judge people you do not even know.