A sit-in is a form of demonstration in which protesters occupy an area to disrupt its operations (for example, by sitting down and refusing to move, hence the name), usually as a means to have demands met.
Sit-ins are effective because they bring a great deal of attention to the protesters' cause. Because authoritative responses often involve police brutality, a sit-in is effective in gaining public sympathy for the protestors.
Sit-ins were used often during the Civil Rights Movement.
Sit-ins are one of the primary tactics of Occupy Wall Street.