Most-favoured-nation and national treatment are the two non-discrimination rules of the WTO

Background

The World Trade Organization's agreements are long and technical, but a few simple principles run through all of them. The first is trade without discrimination, and it has two halves.
Under most-favoured-nation (MFN) treatment, a member cannot normally discriminate between its trading partners: grant one of them a special favour, such as a lower customs duty rate for one of their products, and the same must be given to all other WTO members. MFN is the first article of the General Agreement on Tariffs and Trade (GATT), which governs trade in goods, and it is also a priority in the General Agreement on Trade in Services (GATS) and in the Agreement on Trade-Related Aspects of Intellectual Property Rights (TRIPS), though each agreement handles it slightly differently.
Some exceptions are allowed under strict conditions: a free trade agreement applying only to goods traded within the group, special market access for developing countries, and barriers against products traded unfairly from specific countries.
The second half is national treatment: imported and locally produced goods should be treated equally once the foreign goods have entered the market, and the same applies to foreign and domestic services and to foreign and local trademarks, copyrights and patents. Because it bites only after entry, charging customs duty on an import is not a violation of national treatment even where the like domestic product bears no equivalent tax.
Predictability is delivered by binding. When members agree to open their markets they bind their commitments, and for goods those bindings amount to ceilings on customs tariff rates. A member may apply a rate lower than its bound rate, but can change the binding itself only after negotiating with its trading partners, which could mean compensating them for loss of trade.
Since GATT's creation in 1947-48 there have been eight rounds of trade negotiations, and the agreements also allow change to be introduced gradually through progressive liberalization, with developing countries usually given longer to fulfil their obligations.