In short, it is the buying and selling of human beings with the intent to exploit them, in order to make large profits or obtain some kind of free service or labour.
The legal definition is set out in Chapter 2 of the Prevention and Combating of Trafficking in Persons Act 7 of 2013, but in essence three elements are required to be present and proven in order to have a case of Human Trafficking: Act, Means and Purpose. The latter is the exploitation, or intent to exploit, and comes in many different forms.
In the case of children, only the Act and Purpose are required to prove a case of Child Trafficking.