Romans treated slaves in a variety of different ways. For some, they were a valuable asset, for others they were treated harshly and would never experience freedom.
Types of slaves were; gladiators, cosmetics slaves (), miners, farm hands, nannies, wet-nurses,
The act of freeing a slave was called manumissio, from manus, "hand" (in the sense of holding or possessing something), and missio, the act of releasing. The freeing of slaves was often time a public ceremony performed before some sort of public official, usually a judge. The ceremony would include the owner touching the slave's head with a staff, and after such, the slave was free to go.
However, sometimes, simpler methods were used. Either via the owner exclaiming the slave was free to go, via a will or even having the owner simply invite the slave to sit and recline with them at dinner time.
Slaves were freed for various of reasons, either because of good deeds or out of friendship and respect. Sometimes a slave could buy their freedom, however most often than not, this wouldn't be until the slave was no longer able to work. And the owner would require the slave to purchase their replacement and thus left to live on charity for the remainder of their days.
When slaves were freed they became part of the Freedmen class (Liberti) and their sons would be considered Roman citizens.
They would become patrons to their former Dominus.
Sources of slaves
Debt, prisoners of war, parents selling their children into slavery, born into slavery, criminals.