A warrior in the most simplest of terms, is someone who fights, more specifically fights in a war. Generally, warriors fight for something they believe in with strong conviction, though a few cases throughout history are found of mercenary warriors for hire.
A warrior may fight phiscally, like an army soldier, a 17th century American Indian protecting his home, or the famous Japanese Samauri. But a warrior may also fight with words, like an impassioned journalist or blogger, or may fight with spirit.
Christianity has long recognized the importance of being a warrior: "Christian soldiers marching off to war, with the cross of Jesus going on before..." and Warriors of God.