I think one sometimes leads to the other, but the smuggling, in my specific experience of seeing it, may be a payment to get somebody to what is purported to be a better life. Sometimes that better life does end up being human trafficking—not always—in labour or sex. The trafficking in humans for sex and labour quite bluntly is exactly that: it's slavery of people who are working in trades and not being paid.
On May 1st, 2013. See this statement in context.