Not all slaves were from Africa. Slavery existed among some Native American groups and in some Asian countries, and Europeans were sometimes enslaved by the Ottoman Empire. Russian serfdom is also a variety of slavery. In 1861 when they were emancipated, there were more than 22 million serfs. Slaves were also common in the ancient world - the Greeks, Romans and, Egyptians all held slaves.
Slavery nowadays exists in many forms, from labour and sex trafficking to debt bondage where people are forced to work off small loads. The British Empire outlawed slavery in 1833, but it never really went away, emerging in different forms such as child labour. In the early 1840s, the British declared that slavery in India was over as part of the 'kindness' of British rule, but actually, the definitions were just changed - people were called labourers or servants rather than slaves. Obama recently said about slavery "It's the migrant worker unable to pay off the debt to his trafficker," he said. "The man, lured here with the promise of a job, his documents then taken, and forced to work endless hours in a kitchen. The teenage girl, beaten, forced to walk the streets."
An average slave today costs around $90 (£56) according to a charity, Free the Slaves. In 1850 a slave cost the equivalent of £25,000 in today's money. In the 18th Century, metal bracelets called manilas were brought to West Africa. They were used to barter for slaves and are now only worth around £5 each.
It is now estimated that in 30 years time, slavery could be completely eradicated from society, with the help of governmental and non-governmental organisations and their dedicated work. However it will cost around £6.8bn to get everyone out of slavery and keep them "slave proof".