Human trafficking is the movement of people by means such as force, fraud, coercion or deception, with the aim of exploiting them. It is modern day slavery.
This exploitation includes sexual exploitation, forced labour, criminal exploitation, domestic servitude and organ harvesting. Trafficked people have little or no choice in what happens to them and often suffer abuse due to violence and threats made against them or their families. In effect, they become commodities owned by traffickers, used for profit.
Human trafficking is a crime. It does not always involve crossing international borders.
Find the UN definition here.