A Source Country is a country that girls are trafficked out of. These are also sometimes called "countries of origin. Generally speaking, source countries are very poor, and their social and governmental infastructure might be broken down by years of corruption, war, social unrest, or natural disasters. A source country might also be a place with deep ethnic divisions or culturally-supported racism, which puts minorities at great risk for being trafficked. Traffickers lure people away with false job offers and marriages, the promise of an education, or even kidnap them outright. Examples of source countries include Nepal, Guatemala, the former Soviet territories, and Nigeria.
A Transit Country is a country in which people being trafficked are held temporarily on their way to their final destinations. They are convienent places for traffickers to stop because they are often located at a crossroads between countries that play a more major role in the flesh trade. Some examples of transit countries are Mexico and Israel.
A Destination Country is the country in which trafficked persons finally end up. They are usually wealthy, with a population that has enough disposable income to buy the traffickers' "products". Destination countries include Japan, India, much of Western Europe, and the United States.
Of course, a country can hold more than one of these labels. Many countries with trafficking problems have people being sold abroad as well as domestically, which makes them an example of all three. However, knowing the definitions of each type makes it a little easier to understand how human trafficking works on an international level.