Captain Britain is the defender of Britain in the Marvel Universe and is in many ways the British equivelent of Captain America, however the two characters are very different in origins and ultimate place in the world - with Captain Britain being only one of many such defenders of Britain chosen via a mystical intervention of sorts by Merlyn and his daughter Roma.
Captain Britain is devoted to upholding the laws and customs of Britain and defends it against all threats, at first he fought villains of an earthly nature but as the series progressed he would start to fight much more powerful otherworldly threats such as Jim Jaspers and Necromon.