What are they?

• Economic integration takes the form of the Preferential Trade Area, Free Trade Area, Customs Union, Common Market and Economic Union.
• A preferential trade area (PTA) is a trading bloc that gives preferential access to certain products from the participating countries. This is done by reducing tariffs but not by abolishing them completely. A PTA can be established through a trade pact. It is the first stage of economic integration. 
• A free trade area is the region encompassing a trade bloc whose member countries have signed a free-trade agreement (FTA). Such agreements involve cooperation between at least two countries to reduce/abolish trade barriers. e.g. South Asian Free Trade Area (SAFTA). 
• A customs union is defined as a type of trade block which is composed of a free trade area with no tariffs among members and a common external tariff. e.g. BENELUX (Belgium, Netherlands and Luxembourg).
• A common market has the same features as a customs union, but, in addition, factors of production (labour, capital and technology) are mobile among members. Restrictions on immigration and cross-border investment are abolished. e.g. European Common Market (ECM). 
• An economic union is the last step in an economic integration process. In addition to free movement of goods, services and production factors, it also requires integration of economic policies, both monetary and fiscal. Under an economic union members harmonized monetary policies, taxation and government spending. (e.g. European Economic Union)

Why in News?

• Bangladesh signed its first Preferential Trade Agreement (PTA) with Bhutan allowing duty free access to a range of goods between the two countries. This is the first PTA Bangladesh has signed with any country in the world.
• In 1971, Bhutan was the first country in the world, followed by India as the second, to recognise Bangladesh as an independent country.
• Under the PTA 100 Bangladeshi products will get duty free access to Bhutan. At the same time, 34 items from Bhutan will get duty free access into Bangladesh. Further items can be added in the list later on the basis of discussion between the two countries.

Reference:

1. http://newsonair.com/News?title=Bangladesh-signs-first-Preferential-Trade-Agreement-(PTA)-since-its-independence&id=405623