The Gatherers were Acamarian marauders who lived in semi-autonomous nomadic bands and survived off piracy. Unlike other Acamarians, they maintained their clan identities and practiced elaborate facial tattooing and/or scarification.
In the 2260s, generations of brutal clan warfare on Acamar III was coming to an end. However, a number of clans refused to accept the nascent peace and left the planet, becoming the Gatherers. Over the next century, the Acamarian government made several attempts to bring the Gatherers back into the fold, none of them unsuccessful.
After a century of wandering, the Gatherers had become a major disruption in several sectors near Acamar. Their massacre of a Federation outpost in 2366 compelled Captain Jean-Luc Picard to contact Acamarian Sovereign Marouk and urge her to make another overture to the Gatherers. Negotiations between Marouk and Gatherer leader Chorgan were fruitless at first. But ultimately an assassination attempt on Chorgan by Yuta, Marouk's servant and a member of a rival clan, exposed the poisonous legacy of old hatreds and effected a breakthrough. The Gatherers agreed to a truce and began their reintegration into Acamarian society. (TNG: "The Vengeance Factor")