The caste system was a transformation from the older class system in which an idividual and family's fortune rose and fell over the generations. The castes were legally fixed as hereditary with sumptuary laws to define the lifestyles each caste.

The Tokugawa regime also removed the Samurai from the land as a preventive measure against the posibility of formation of independent warrior bands in the provinces. Instead the Samurai lived in the precints of the castle towns of thier lords, receiving stipends in rice.

The psychological compensation was that the martial arts developed into a very
theatrical ,philosophically comprehensive systems of both mental and moral training.

Were not only warriors but also gave patronage to culture and education. They sometimes became scholars and physicians or taught Confusian classics to children in Primary schools or home tutoring. Many entered religious orders. Mr. Cleary mentions the Renzai order of Zen Buddhism as a primary example.

There were 4 main caste:

Warrior
Farmer
Artisan
Merchant

with two outcasts:

Polluted Ones
Inhumans

While wealthy merchants could and probably did secure their family's position through marriges or adoptive alliances and free people could be outcasted for crimes the four casts were kept distinctively separated generally.

According to the introduction of "Code of the Samurai" by Thomas Cleary, the 'Samurai class originated as an offshoot or specialization of the aristocracy.' Since only one son inherited everything (full privelidges from the father) that meant all the other sons had to have some means of living, usually becoming warriors or monks.

Originally samurai (attendants) formed the armed guard of the aristocracy. By the time they were in control of Japan, they began to label themselves as bushi or warrior/knights to distance themselves from their former 'attendant' status. By the height of their power, the had built for themselves a new station in raising the station of their caste. If I recall correctly (I'll have to double check) they had at least 5 classes of stations.