On entry to the WTO, each member negotiates a schedule of concessions setting out specific commitments on trade that it will provide to other WTO members. For example, it may commit to impose maximum tariffs of x% on certain goods or to allow a provider from another member state to provide certain services which were previously reserved to nationals of the first member state.
The UK is already a member of the WTO but its commitments are set by the European Commission acting on behalf of all EU member states; this is because (a) EU member states agree to give the Commission the lead role in trade policy; and (b) the EU Customs Union necessitates the use of a "Common External Tariff" for goods i.e. the same import duty is payable when goods enter the EU, whether the point of entry is the UK or another EU member state. The UK will therefore require its own UK-specific set of commitments when it leaves the EU (see below).
Each member of the WTO agrees to provide equal trading terms including lower tariffs and customs duties to all WTO members in the areas covered by its commitments; this is known as the most-favoured-nation (MFN) principle. In reality, many nations use their MFN tariff commitments as an import duty ceiling and the actual tariffs imposed on WTO members in practice are often set at a lower level.
National treatment is the obligation to accord the same treatment to goods/services of other WTO members as the WTO member accords to goods/services provided by its own nationals (again, in the areas covered by its commitments).
However, a WTO member's commitments will often contain a significant number of exceptions and derogations from these principles, particularly in relation to services and certain categories of goods such as agricultural produce. In relation to services, the GATS provides for commitments to be made in relation to 4 different "modes" of service provision (see below); these too are often subject to significant derogations. Indeed, in some cases, WTO members will specify that they are "unbound" i.e. they make no commitment at all in relation to the type of goods or services specified in the schedule.
Free trade agreements (FTAs) are an exception to the usual MFN rule. They allow WTO members to go further than their WTO commitments but without having to offer that preferential treatment to all other WTO members (it would only be extended to the other party to the FTA). It is also increasingly common for countries to have bilateral trade facilitation agreements relating to issues such as customs formalities (see below).