A Free Trade Area is when a group of countries agree to remove barriers to trade between members
A Customs Union both eliminates barriers to trade between countries and also has a common external tariff against non-member states
Customs Union benefits and limitations:
Trade creation occurs when a customs union results in high-cost domestic production being replaced by imports from a more efficient source within the customs union
Trade diversion occurs when a customs union results in trade switching from a low-cost supplier outside the customs union to a less efficient source within the customs union