1) An interest group is an organization that works to influence government policy in order to achieve shared goals for its members. Interest groups differ from political parties, which seek to get members elected to make policies.
2) Interest groups are part of "iron triangles" that influence policymaking along with congressional committees and government agencies. Small interest groups tend to be more effective than large ones due to problems with free riders and spreading benefits.
3) Interest groups use lobbying of lawmakers, electioneering through campaign donations, litigation, and generating public support to influence public policy and shape political agendas. Their role is to provide information to lawmakers and be policy experts and strategists.