Information below is from here http://en.wikipedia.org/wiki/Ronin
A rōnin was a masterless samurai during the feudal period (1185–186 cool of Japan. A samurai became masterless from the ruin or fall of his master, or after the loss of his master's favor or privilege. Since a rōnin did not serve any lord, he was no longer a samurai, as the noun samurai came from the verb saburau which was the Japanese for "to serve".
History
In the Kamakura and Muromachi periods, when warriors held lands that they occupied, a ronin was a warrior who had lost his lands. During these periods, as small-scale wars frequently occurred throughout Japan, the daimyo needed to augment their armies, so ronin had opportunities to serve new masters. Also, some ronin joined in bands, engaging in robbery and uprisings.
Especially in the Sengoku period, daimyo needed additional fighting men, and even if one's master had perished, a ronin was able to serve a new lord. In contrast to the later Edo period, the bond between the lord and the samurai was loose, and some samurai who were dissatisfied with their treatment left their masters and sought new lords. Many warriors served a succession of masters, and some even became daimyo. As an example, Tōdō Takatora served ten lords. Additionally, the division of the population into classes had not yet taken place, so it was possible to change one's occupation from warrior to merchant or farmer, or the reverse. Saitō Dōsan was one merchant who rose through the warrior ranks to become a daimyo.
As Toyotomi Hideyoshi unified progressively larger parts of the country, daimyo found it unnecessary to recruit new soldiers. Next, the Battle of Sekigahara resulted in the confiscation or reduction of the fiefs of large numbers of daimyo on the losing side; in consequence, many samurai became ronin. As many as a hundred thousand ronin joined forces with Toyotomi Hideyori and fought at the Siege of Osaka. In the ensuing years of peace, there was less need to maintain expensive standing armies, and many surviving ronin turned to farming or became townspeople. A few, such as Yamada Nagamasa, sought adventure overseas as mercenaries. Still, the majority lived in poverty as ronin. Under the third Tokugawa shogun Iemitsu, their number approached half a million.
Initially, the shogunate viewed them as dangerous, and banished them from the cities or restricted the quarters where they could live. They also prohibited serving new masters. As ronin found themselves with fewer and fewer options, they joined in the Keian Uprising. This forced the shogunate to rethink its policy. It relaxed restrictions on daimyo inheritance, resulting in fewer confiscations of fiefs; and it permitted ronin to join new masters.
Among the most famous ronin are Miyamoto Musashi, the famed swordsman, and the Forty-seven Ronin.
Not having the status or power of employed samurai, ronin were often disreputable, and the group was a target of humiliation or satire. It was undesirable to be a ronin, as it meant being without a stipend.
As an indication of the humiliation felt by samurai who became ronin, Lord Redesdale recorded that a ronin killed himself at the graves of the Forty-Seven Ronin. He left a note saying that he had tried to enter the service of the daimyo of the Chōshū Domain, but was refused. Wanting to serve no other master, and hating being a ronin, he had decided to kill himself.
On the other hand, the famous 18th century writer Kyokutei Bakin renounced his allegiance to Matsudaira Nobunari, in whose service Bakin's samurai father had spent his life. Bakin became voluntarily a ronin, and eventually spent his time writing books (many of them about samurai).
