The slave trade was at its most brutal and organised during colonialism. In the slave trade across the Atlantic Ocean, European goods such as weapons and cloth were sent to Africa in exchange for slaves.
The slaves, in turn, were taken across the Atlantic to the Americas in exchange for cotton, sugar or tobacco. 10-12 million people were taken from Africa as part of the transatlantic slave trade. The painting in the background was done in 1830 by Johann Rugendas and shows the cellar of a slave boat bound for Brazil.
UNESCO.org section on the Slave Trade.
Encyclopaedia Britannica entries on Slavery and Slave Trade
United Nations Interregional Crime and Justice Research Institute section on Human Trafficking