African slaves were bought cheaply in African countries, and shipped into the Caribbean in the hold, in much the same way as other commodities. They were not only kept by those with large amounts of land, but also by small farmers, and city dwellers.
It is generally believed that the economics of large acreages of Cotton and Sugar, combined with low levels of servant indenture, meant that America came to rely upon slaves to work the land. It would have been impossible to farm the vast amounts of these goods without slave labor, as liberated workers would have demanded pay, and probably better working conditions and fewer hours; slaves, on the other hand, were not employees, and could be worked in extreme conditions. African Slaves were able to repopulate, like livestock, and were also supposed to be immune to Malaria and other diseases (Racism saw them as stronger, and better suited to physical labor than white people); unlike Indian slaves, black people (at least in the early to mid 18th Century) stood out among the general population.