The Customs Union

A customs union is a form of trade agreement between two or more countries. It means they decide not to impose tariffs (taxes on imports) on each other's goods and agree to impose common external tariffs on goods from countries outside their customs union

However

A single market is different because it is a deeper form of integration than a customs union.

A single market involves the free movement of goods and services, capital and labour.

In addition to a common external tariff, a single market also tries to cut back on the use of non-tariff barriers such as different rules on product safety and environmental standards replacing them with a common set of rules governing trade in goods and services within the common market.