The 2370s armed resistance of Federation colonists left on the Cardassian side of the border, who fought the Cardassians and clashed with Starfleet.
After the Federation-Cardassian treaty of 2370 set a new border, some Federation colonies ended up in Cardassian territory or in the Demilitarised Zone. Many colonists refused to leave, and when they felt the Cardassians were arming against them, some formed the Maquis, a resistance group that fought back with raiders and sabotage.
The Maquis are introduced in the Deep Space Nine two-parter The Maquis, in which Sisko's friend Commander Cal Hudson turns out to be one of their leaders. In The Next Generation, Ro Laren goes undercover among them and then defects to their side. Later, Starfleet security officer Michael Eddington betrays Sisko and becomes a Maquis leader.
In Voyager, Chakotay's Maquis cell, including B'Elanna Torres, is stranded with Voyager in the Delta Quadrant and merges with its crew. After Cardassia joined the Dominion in 2373, the Jem'Hadar destroyed the Maquis in the Alpha Quadrant, a loss Voyager's former Maquis learned about through letters from home.