There are 2 unit systems in the world : the metric system (metres, grams, liters etc), and the imperial system (miles, pounds,gallons etc)
The metric system is used in most of the world. However, the USA uses the imperial system, and the UK uses a mix of imperial and metric units.
To make things even more confusing, the US imperial unit system is sometimes slightly different from the UK imperial unit system.
For example, the US gallon (a unit for volume) is not exactly the same as the UK Gallon :
1 US gallon = 0.83 UK gallon (= 3.79 liters)
This lesson's goal is to familiarize yourself with both unit systems and with the language of units and measurements.