Kiddushin, Chapter 1, Mishnah 2. This mishnah deals with the ways in which an eved Ivri - a Jewish servant - enters into the relationship with his master, and the ways in which he leaves it.
The ways an eved Ivri is acquired:
The mishnah opens: "Eved Ivri nikneh b'kesef u'vishtar" - a Hebrew servant is acquired with money and with a document.
"B'kesef" - acquisition through the payment of money. If he sells himself, the master pays the money directly to him; and if the court sells him, the master pays the court. The court sells a person who stole and has no money with which to repay.
"U'vishtar" - acquisition by means of a document. The servant would write to the master: "I am hereby sold to you" or "I am hereby acquired by you," and that alone was enough to make him an eved Ivri.
The ways he goes free:
The mishnah continues: "V'koneh es atzmo b'shanim u'vayovel u'vigra'on kesef" - and he acquires himself through years, through the Yovel and through a deduction of money: three ways in which he leaves the relationship.
"B'shanim" - at the completion of the term of service. The court may sell him for six years. Regarding one who sells himself, the Gemara records differing opinions, and according to one opinion he may sell himself even for more than six years, and he goes out only when the stipulated conditions have been fulfilled.
"U'vayovel" - if the Yovel year falls within the term of service and interrupts it, the servant goes free.
"U'vigra'on kesef" - returning part of the money that was paid. How so? He was sold for six years for six hundred dollars and worked three years, so that three hundred dollars' worth of labor remains for the three remaining years. When he pays the master those three hundred dollars, it is as though he has purchased his freedom with them.
An amah Ivriyah:
There is another category of servitude, which applies to a female: a young girl who is sold into this servitude is called an amah Ivriyah. She too is acquired with money and with a document, and she too goes out through years (at the end of the term, six years), through the Yovel which interrupts the term, and through a deduction of money - that if she was sold for six years for six hundred dollars and three years remain, three hundred dollars are paid and she goes free.
However, "yeseirah alav amah Ivriyah shekonah es atzmah b'simanim" - an amah Ivriyah has an additional feature beyond his: she acquires herself through signs. In addition to all these ways, she has a further route to freedom: the signs of maturity. Once she has produced two hairs, this is proof that she has reached maturity and become a na'arah, and she immediately leaves the status of an amah Ivriyah. This is a unique way of going out that has no parallel in an eved Ivri.
The nirtza:
A nirtza is an eved Ivri who, at the end of his term of service, decides to remain in the relationship with his master, and his ear is pierced in the manner described in the Torah. The mishnah rules that he is "nikneh birtziah" - acquired through piercing: in his new status as a nirtza he is acquired by the piercing of his ear.
And how does he go out? "V'koneh es atzmo bayovel u'vemisas ha'adon" - and he acquires himself through the Yovel and through the death of the master:
Through the Yovel - the Torah describes how, when the Yovel year arrives, he leaves the relationship. Until then he remains a nirtza, however many years remain until the Yovel.
Through the death of the master - when the master dies, the nirtza goes free and does not serve the son or the sons. In this he differs from an ordinary eved Ivri, for an eved Ivri does serve the son when the master dies; however, the halachah is that he serves the son alone, and if there is another heir besides the son, he does not work for him.
Although the law of a nirtza does not apply to an amah Ivriyah, and she never continues in the relationship, she resembles the nirtza in this point: when the master dies she goes free, and she does not serve the son or any other heir.
In summary: in this mishnah we learned the ways in which an eved Ivri enters and leaves servitude - he is acquired with money and with a document, and he acquires himself through years, through the Yovel and through a deduction of money; the distinctive feature of an amah Ivriyah, who acquires herself also through the signs of maturity; and the law of the nirtza, who is acquired through piercing and acquires himself through the Yovel and through the death of the master, unlike an eved Ivri, who serves the master's son.