Global trade, also known as international trade, is simply the import and export of goods and services across international boundaries.
Goods and services that enter into a country for sale are called imports. Goods and services that leave a country for sale in another country are called exports. For example, a country may import wheat because it doesn't have much arable land, but export oil because it has oil in abundance.
A fundamental concept underlying global trade is the concept of comparative advantage, developed by David Ricardo in the 19th century. In a nutshell, the doctrine of comparative advantage states that a country can produce some goods or services more cheaply than other countries. In technical terms, the country is able to produce a specific good or service at a lower opportunity cost than others.
An opportunity cost is the benefit one gives up in making an economic choice. The classic example is guns and butter - domestic investment over defense spending. The more guns you produce, the less funds are available to invest in public schools and infrastructure, for example. The more you invest in the domestic economy, the less you can spend on defense.