The Ultramarines are rigid adherents to the Codex Astartes, written by their Primarch to make sure his knowledge of warfare was preserved. They are the archetypal Space Marine Chapter from whose stable genetic material most new Founding chapters are formed. In the aftermath of the Horus Heresy their Primarch became the first High Lord of Terra and used the Ultramarines to restore the shattered Imperium. The Ultramarines are based loosely around ancient Rome, though they also use imagery relating to Classical Greece.
They are the 8th Legion to be created by the Emperor
Their Primarch is Roboute Guilliman who's world is Macragge