Since the Aryan society contained not only, the descendants of ensuing migrants from Central Asia, but also as many as three or four other native ethnic races, the Aryan priests simplified the society by the dividing the society into four castes.

Brahmins, were to administer the scriptures in rituals.
Kshathriyas, ruled the land, protected the people
Vaishyas, comprised of merchants, cattle herders and artisans.
Shudras, were servants and un-free peasants.

Your caste is what you are born into. If you have been born into a lower caste, then you can never rise above it, to perform other functions usually done by upper caste people.