Under the World Trade Organisation (WTO) agreements, countries cannot normally discriminate between their trading partners. If any country grants one country a special favour such as a lower customs duty rate for one of their products the same would need to be extended to all other WTO members. This principle is known as most-favoured-nation (MFN) treatment.
MFN is so important a principle that it is the first article of the General Agreement on Tariffs and Trade (GATT), which governs trade in goods. MFN is also a priority in the General Agreement on Trade in Services (GATS) (Article 2) and the Agreement on Trade-Related Aspects of Intellectual Property Rights (TRIPS) (Article 4). Together, those three agreements cover all three main areas of trade handled by the WTO.
Some exceptions, however, are allowed under WTO regime. For example, countries can set up a free trade agreement that applies only to goods traded within the group — discriminating against goods from outside. Or they can give developing countries special access to their markets. Or a country can raise barriers against products that are considered to be traded unfairly from specific countries. And in services, countries are allowed, in limited circumstances, to discriminate. But the agreements only permit these exceptions under strict conditions. In general, MFN means that every time a country lowers a trade barrier or opens up a market, it has to do so for the same goods or services for all its trading partners whether developed or developing.