The Maquis were a paramilitary organization that existed for a short time in the 2370s.
Following the Federation-Cardassian peace treaty of 2370, a number of Federation and Cardassian colonies were located on the 'wrong' sides of the new border. After an abortive attempt at evacuating these colonies, the decision was made to enclose them in a Demilitarized Zone.
The Federation colonists who found themselves under Cardassian rule in the Demilitarized Zone reported persecution by Cardassian forces (whether these were initially the Cardassian Union government or the Cardassian colonists remains unclear). In response, they formed a resistance organization, which they named the Maquis (after the French Resistance of Earth's World War II period, 1939-1945), and engaged in a private war against the Cardassian colonies within the Demilitarized Zone.
The Maquis was partially made up of former Starfleet officers who had either family ties to the colonies or strong sympathies for their cause, and in some cases were illegally working from within Starfleet itself. Its ships (primarily Peregrine Attack Fighters) and other supplies were often of Federation or neutral origins, provided by sympathetic Federation citizens, and delivered through a Starfleet blockade.
The Maquis were always at a disadvantage, as the Cardassian colonies were secretly being supported by the Cardassian Union. In order to partially balance out this disadvantage, Maquis forces often took shelter and established their bases of operations in the Badlands, an area of turbulent space storms near Bajor. (In fact, the famous voyage of U.S.S. Voyager began with a mission to disrupt Maquis operations in the Badlands.)