A marginalized community is a group that’s confined to the lower or peripheral edge of the society. Such a group is denied involvement in mainstream economic, political, cultural and social activities. Marginalization or social exclusion deprives a group of its rightful share of reach to productive resources and ways to utilize its maximum potential for prosperity. It’s directed at groups who are seen to differ from perceived norms.
Marginalization can affect a community at the macro-level, where its members may lack access to affordable formal education, equal employment or gender-related opportunities. At this level, victims are denied influence or access to the official power structure and can’t participate in decision-making processes. At the micro-level, exclusion is manifested in income discrepancies, occupational status, and social networking around race, religion or gender.
Social exclusion leads to subordination of one section of the society as another exercises economic, social and political dominance. Affected groups are usually socially illiterate, uneducated and reliant on others. They’re poor and lack basic necessities of life, including access to good healthcare. Marginalized groups include the poor, working children, gender inequality, the disabled and persons speaking a minority language.
Here's another description according to the National Gender and Equality Commission in Kenya.