Caste Systems predate the novel Brave New World by hundreds of years. Huxley mainly based the book's caste system on the one in India. So, they are very real, not hypothetical.
The highest caste in India is the Brahmin (teachers, scholars, priests), which match up with the Alphas, more or less. There's four more castes under this, and then there's the outcasts, like those on the Savage Reservation.
In Elizabethan England there was a kind of caste system called The Great Chain of Being, which divided society up by rank and privilege. The King was on top, followed by the nobility and dukes--all the way down to the lowest classes (witches, heretics, insane).
Most caste systems are very rigid: you are born into them, and there's little mobility. There's really no formal caste system in the U.S., although the socio-economic class "ladder" is close.