From A Wiki of Ice and Fire
The Andals are one of the three major ethnic groups of Westeros. Their arrival caused the fall of many of the kingdoms of the First Men which had developed in the wake of the Pact with the children of the forest and brought the Faith of the Seven to the Seven Kingdoms.
The Andals originated in the Hills of Andalos in the eastern continent, where legend tells that they were visited by the incarnations of the Seven, who crowned their first king, Hugor of the Hill. They learned the art of working iron from the Rhoynar who lived along the Rhoyne.
About six thousand years ago, the Andals sailed west into Westeros, first landing in the Fingers and attacking the Vale. The original Andal invaders painted and carved the seven-pointed star of the Seven on their bodies. They destroyed the weirwood trees and killed the children of the forest that they came across. A hill, now known to the Westerosi as High Heart, was sacred to the children of the forest. There the Andal king named Erreg the Kinslayer cut down the children’s grove of thirty-one weirwoods.  It is said that the First Men killed half of the children of the forest with bronze blades, and the Andals finished the job with iron. 
After several hundred years of war, the Andals conquered the six southern kingdoms of the First Men and created their own. Only the Kings of Winter resisted the Andals successfully, stopping them at Moat Cailin. 
Andal culture is marked by the strong influences it has had on the culture of the Seven Kingdoms. The Andals brought the Faith to Westeros and with it the warrior tradition of the knight. The Andals also brought their own language, which became the Common Tongue of Westeros even in the North. The Andals also brought a superior writing system that supplanted the First Men's runes.