Andals

The Andals are a race of men who invaded Westeros six thousand years prior to the events of the series. They overran and conquered most of the continent, aside from the North which remained independent. The Andals brought knighthood, chivalry and iron-wrought weapons and armour with them from Essos.

Most of the current population of Westeros is of Andal descent. The exceptions are the North, where the blood of the First Men is still strong; the Iron Islands, where the First Men inhabitants and Andal invaders have intermingled to form the modern ironborn; and Dorne, where the Andal inhabitants have intermingled with the later Rhoynar refugees from the east.

In the books
In the Song of Ice and Fire novels, the Andals were originally the inhabitants of a region known as the Hills of Andalos, located south of the modern Free City of Pentos. The Andals were given a vision of a god manifesting to them as seven robed figures, and were bade to leave their homeland and conquer Westeros, which they did in the Seven's name. Whilst following the code of chivalry and honour, the early Andals were ruthless in conquering and destroying those who opposed the Faith of the Seven and the justness of their cause.

The Andals' blood remains purest in the Vale of Arryn, where their ships landed and commenced the invasion of the continent. The Andals' spoken language and written alphabet have supplanted the rune-based Old Tongue of the First Men, even in the North.