Slavery is an abhorrent institution of which the Bible heartily approves in passages which advise slave owners not to be too hard on their chattel. Other holy books of various religions also approve of it. Despite this approval, the vast majority of Christians, as well as almost every other member of the Human race, find the practice reprehensible (Ahh, the irony!). Slavery is the ownership of one human being by another for the owner's benefit. See What the Bible says about slavery. The Bible implies that the relationship of wife to husband bears many of the characteristics of the relation between slave and master. How's that for family values! The picture at right is the cargo plan of a slave ship, about three hundred feet long with a cargo of 300-800 slaves. The survival rate for the eight-week transatlantic ship was 66%.
Slavery in 19th Century United States
Certain slave-owning Southern States threatened to secede from the Union if the Republican candidate Abraham Lincoln was elected president because his anti-slavery views were well-known. (Very sadly the Republicans didn't stay as good as they were at the time of Lincoln.) Lincoln was elected, Southern States seceded, and the American Civil War was fought afterwards as a result. (The North won and the 13th Amendment ended slavery in the United States.) Conservatives will go to any lengths to preserve the advantages the wealthy enjoy, including starting wars which result in the deaths of hundreds of thousands of people (mainly poor). Sound familiar?
If someone is descendant of slave owners in the South, they are probably also a descendent of slaves. 1/3 of White Americans have some Black ancestry. Descendants of slave owners have a family that goes (a) far back, (b) in the south. These people are likely in that one third, so slave owner ancestry implies likely slave ancestry. The states where slavery was allowed before the American Civil War are now the backbone of the Bible Belt.
In North America black people and Native Americans were enslaved but overseers and slave owners regularly took advantage of female slaves so there were Mixed-race slaves as well. Mixed race slaves tended to be kept as house slaves so sometimes white men would use female slaves sexually over several generations and Babies that were predominantly white were occasionally born to slave mothers.
Treatment of slaves
If slaves wanted to run away this was called drapetomania, whipping was the prescribed punishment or therapy,  alternatively amputating the toes of a slave would physically prevent that slave running away. 
The links below shows how even relatively considerate slave owner treated slaves.
Cruel slave owners and overseers were very much worse.
Slavery under Islam
In Islammic countries white Christians were enslaved as the Koran allows Muslims to enslave Non-Muslims. See Problems with Muslim Sharia Law The Barbary Pirates were notorious for enslaving Christians.
In the 21st Century slavery is officially illegal almost everywhere but it exists with different names. A side effect of this is that today all slave owners are criminals and all slaves are at the mercy of criminals.
After the American Civil War freed slaves were so grateful that they called Abraham Lincoln their father. There are captives today and we who are free can help them. A small effort with a pen or a computer keyboard can transform people's lives. The captives probably won't know it was you who helped free them but they will be grateful to those who helped free them for the rest of their lives.
Men women and children are bought and sold and forced to work for hardly any pay or for no pay at all and depend on their owner or employer’s mercy.
Example of modern slavery
- Madagascar maids: Misery in the Middle East Women are duped into going to the Middle East and believe they will get good jobs. When they get there their papers are taken away so they are trapped and their lives become a living hell.
- Almaz's story, the disturbing story of a young Ethiopian woman abused by her employers in Saudi Arabia
- A case of modern day slavery in the suburbs There is even slavery in liberal cournties like the UK, sometimes the police take slavery allegations seriously but not always, see Metropolitan Police pays damages over slave case women.
- Woman Enslaved Through Arranged Marriage Arranged marriages can amount to slavery.
- Why are so many of the UK's missing teenagers Vietnamese? Vietnamese children are trafficked into the UK, they are forced to work in Cannabis farms and if they run away or don't return to the traffickers their families in Vietnam may be harmed.
- The women who come to the UK as brides but end up as slaves Marriages can also become slavery.
- Why India's brick kiln workers 'live like slaves' Indian brick kiln workers live and work in unhealthy conditions, those who try and run away may have their hands amputated.
- 'Oliver Twist' children used in crime, warns anti-slavery commissioner Children in the UK are manipulated into committing crime.
Differences between slavery and other human rights abuse
- Slaves are forced to work e.g. though threats of beatings and/or psychological brainwashing
- Slaves are controlled by a master or mistress so they have no independent lives, abuse or threatened abuse is used to control them.
- Slaves are bought and sold like traded goods and not treated as human beings.
- Slaves are physically prevented from going where they like.
*Women from eastern Europe are bonded into prostitution.*Children are trafficked between West African countries and men are forced to work as slaves on Brazilian agricultural estates.*Children are traficked into the UK and forced to work in illegal Cannabis farms. *Contemporary slavery takes various forms and affects people of all ages, sex and race. 
Religious cults sometimes brainwash members so they are willing to work long hours for little or no pay while other cult members work at high pay outside the cult, they live in poor districts spending little of what they’ve earned on themselves and giving most to the religious group.
They (the cult leaders) persuade followers to drop their families, jobs, careers, and friends to follow them. They (not the individual) then take over control of their followers' possessions, money, lives. 
There are persistent rumours of some cult members being physically confined but when a cult cooperates to hide what is going on this is difficult to prove. The Sea Org run by Ron Hubbard of Scientology is considered slavery and is one case where physical confinement could be proved.  In other cases where there is no physical confinement but only psychological pressure it can be hard to say if this is slavery or not but it certainly is serious abuse.
This happens even in liberal countries like the UK and involves weakening a person's psychological resistance so the person is unwilling to resist demands of the master or mistress. People may be groomed over many years with pressure to conform to the master/mistress steadily increasing over time. In the early stages the would-be master or mistress may try and prevent the other seeing how controlling the relationship will later become. There are people in the BDSM community who do take advantage of the submissive elements therein. Word gets out on them and their faces become quite unwelcome within the scene very quickly. We only know those where word got out, we don't always know about those who got away with it. On the Internet it's easier for someone to hide behind different usernames and carry on causing trouble when one username has been outed.