But what are preferential tariffs? Let’s see what wikipedia says about this:
The Generalized System of Preferences, or GSP, is a formal system of exemption from the more general rules of the World Trade Organization, WTO, (formerly, the General Agreement on Tariffs and Trade or GATT). Specifically, it's a system of exemption from the Most Favored Nation principle, MFN, that obligates WTO member countries to treat the imports of all other WTO member countries no worse than they treat the imports of their "most favored" trading partner. In essence, MFN requires WTO member countries to treat imports coming from all other WTO member countries equally, that is, by imposing equal tariffs on them, etc.
GSP exempts WTO member countries from MFN for the purpose of lowering tariffs for developing countries (without also doing so for rich countries). The idea of tariff preferences for developing countries was the subject of considerable discussion within UNCTAD in the 1960s. Among other concerns, developing countries claimed that MFN was creating a disincentive for richer countries to reduce and eliminate tariffs and other trade restrictions with enough speed to benefit developing countries.
According to the US Trade Representative’s web site: The U.S. Generalized System of Preferences (GSP), a program designed to promote economic growth in the developing world, provides preferential duty-free entry for more than 4,650 products from 144 designated beneficiary countries and territories. The GSP program was instituted on Jan. 1, 1976, and authorized under the Trade Act of 1974 for a 10-year period. It has been renewed periodically since then, most recently in 2002, when President George Bush signed legislation that reauthorized the GSP program through 2006.