TEU stands for Twenty-foot Equivalent Unit. Basically it is the unit in which freight is measured during shipments.
The unit is based on the volume of containers. There are several kinds of containers, but 1 TEU is based on a 20 foot long container.
However the height of the containers often varies which is why the unit is rather inexact. Approximately 1 TEU equals 19,26m^3.
But there is another type of container, which is commonly used:
The so-called FEU ( = Forty-foot Equivalent Unit). One FEU has twice the volume of one TEU, so 2 * 19,26m^3 = 38,52m^3. The only difference is the length of the container: A FEU measures 40 foot in length.
If you'd like to know more about the different types of containers, have a look at our article about special equipment.