While there is some disagreement about exactly what constitutes human trafficking, put simply, the subject is about the;

sale and trade of human beings

Human trafficking can occur across international borders but is the trade of people and does not always involve the movement of a person from one place to another.

It is believed that human trafficking represents an estimated $31 billion annually, is the fastest-growing criminal enterprise globally and will or may have already surpassed  trafficking of drugs and arms as the largest criminal enterprise in existence.

Human trafficking is condemned as a violation of human rights by international conventions.

The United Nations Trafficking Protocol or Palermo Protocol, which was entered into force in December 2003 is the first international, legally binding instrument and the only one with an agreed-upon definition. It describes trafficking as: 

(a) [...] the recruitment, transportation, transfer, harbouring or receipt of persons, by means of threat or use of force or other forms of coercion, of abduction, of fraud, of deception, of the abuse of power or of a position of vulnerability or of the giving or receiving of payments or benefits to achieve the consent of a person having control over another person, for the purpose of exploitation. Exploitation shall include, at a minimum, the exploitation of the prostitution of others or other forms of sexual exploitation, forced labour or services, slavery or practices similar to slavery, servitude or the removal, manipulation or implantation of organs;
(b) The consent of a victim of trafficking in persons to the intended exploitation set forth in sub-paragraph (a) of this article shall be irrelevant where any of the means set forth in subparagraph (a) have been used;
(c) The recruitment, transportation, transfer, harbouring or receipt of a child for the purpose of exploitation shall be considered “trafficking in persons” even if this does not involve any of the means set forth in sub-paragraph (a) of this article;
(d) “Child” shall mean any person under eighteen years of age.
— UNITED NATIONS - Office on Drugs and Crime

 There are many different types of trafficking globally and locally.