What is the caste system in Brave New World?

Simply put, the caste system in Brave New World is based on intelligence that was more or less determined during conception. 

Therefore, those of higher castes (and thus higher intellect), were the most nurtured throughout their embryonic development. However, those of lower castes had their mental development actively sabotaged by dividing one embryo multiple times through Bokanovsky's Process and then by adding alcohol or restricting oxygen flow to these fetuses. There are five total castes, all of...

Simply put, the caste system in Brave New World is based on intelligence that was more or less determined during conception. 

Therefore, those of higher castes (and thus higher intellect), were the most nurtured throughout their embryonic development. However, those of lower castes had their mental development actively sabotaged by dividing one embryo multiple times through Bokanovsky's Process and then by adding alcohol or restricting oxygen flow to these fetuses. There are five total castes, all of which are characterized below and organized in descending order.

Alphas: They typically dress in gray and are extremely intelligent. They hold the most sophisticated jobs. They are prone to becoming very individual and to crave more than a strictly structured society can give them (e.g. Bernard and Helmholtz). 

Betas: They usually wear mulberry colored clothes and are intelligent enough to easily perform skilled work, though the work they do is slightly less complex than what Alphas do. As well, they are far less likely to be dissatisfied with what society has to offer them.

Gammas: They most often dress in green and are considered to be of average intelligence. They are moderately skilled laborers, often working as mechanics or in other positions that involve manual labor and a degree of thought. Gammas and the proceeding castes go through Bokanovsky's Process and have their development hindered as a fetus. 

Deltas: They dress in khaki and have very little individuality and intellect. They are employed in simple and repetitive professions such as helicopter attendants or factory workers.

Epsilons: They wear black and are often referred to as "semi-morons" because they lack even the ability to read and write. They do the most menial and unwanted jobs. 

Also, due to the sabotage of embryonic growth, people of lower castes are much shorter and considered uglier than people of higher castes.

It should be noted that all of the above plays into the overall theme of class conflict Huxley is trying to present and satirize.