Human trafficking is, allegedly, modern day slavery. This, at least, is how those vaguely familiar with the term view it. What it really is, however, is an attempt to use fancy words to make old crimes seem even more morally worse than they have traditionally have been considered to be.
The difference between slavery and human trafficking, in my opinion, is that in slavery you coerce people into doing otherwise legal things, like agriculture. It is a status conferred by the government, more or less. Some people are slaves and some people are not and whether they are or not is usually up to somebody else, either the state or somebody appointed or approved by the state.