According to the Star Trek timeline displayed in the museum at the Las Vegas Hilton, the Maquis are a terrorist group of Federation settlers who were displaced as part of a treaty between the Federation and the Cardassian Empire that turned their sector of space into a demilitarized zone.

A present-day analogy might be if the Israelis and the Palestinians finally made peace, on the condition that the Israeli settlers in disputed territories pulled up stakes and left...and the settlers decided they'd rather keep the war going.*

* mawa tells me that a real-life example would be the abandoned French settlers in Algeria during/after the Algerian war. I will have to take his word for this.