From A Wiki of Ice and Fire
The Crannogmen are so called for their habit of living in small villages formed of reeds and thatch that sit atop floating islands in the mire (crannogs). They are a poor people and seldom leave their lands subsisting on fishing and frogging.
They are talented hunters and warriors and despite their short stature and somewhat primitive lifestyle, the crannogmen have proven a notoriously difficult people to conquer. They use nets, bronze knives, frog spears, and round leather shields. They fight with poison, guerrilla tactics, and a mastery of their difficult terrain. According to those who have fought them, their houses, even their castles move. They are disparaged as "frog-eaters, swamp-dwellers, mud-men and bog devils" by most other people of the Seven Kingdoms, and are looked down upon for their tactics, which are viewed as cowardly.
The histories say that the crannogmen grew close to the children of the forest in the days when the greenseers tried to bring the hammer of waters down upon the Neck. Since that time, they have sworn their oaths of fealty to the Starks thousands of years ago. They are ruled by House Reed from Greywater Watch. Other families of crannogmen are House Fenn, Peat, Boggs, Cray, Quagg, Greengood and Blackmyre.
They have an ages-old feud with House Frey, who rule the lands immediately to the south.