Slavery is a form of forced labor where a person is compelled to work for another (sometimes called "the master" or "slave owner").

Slaves are held against their will from the time of their capture, purchase, or birth, and are deprived of the right to leave, to refuse to work, or to receive compensation (such as wages) in return for their labor.
Evidence of slavery predates written records, and has existed to varying extents, forms and periods in almost all cultures and continents. In some societies, slavery existed as a legal institution or socioeconomic system, but today it is formally outlawed in nearly all countries. Nevertheless, the practice continues in various forms around the world.
Freedom from slavery is an internationally recognised human right. Article 4 of the Universal

Declaration of Human Rights states:
No one shall be held in slavery or servitude; slavery and the slave trade shall be prohibited in all their forms.

The word slavery is often used as a pejorative to describe any activity one finds unpleasant or distasteful. On the one hand, this means the word slavery is applied in situations where it does not technically fit the definition. On the other hand, it also means that it is often not applied in situations that do fit the definition, but where the speaker feels that everyone has a duty to perform the action. Examples of the latter might include jury duty or military conscription, where a person is compelled to perform a job and is paid much less than one would have sought for a similar job in a free market.

sources: http://en.wikipedia.org/wiki/Slavery