A Joule is the energy delivered in 1 second by a 1 Watt power source. It is the unit of energy or work.
A Watt is a Joule per second, e.g. a 100 Watt lightbulb uses 100 Joules of energy every second it is on. It is the basic unit of electrical power.
A kiloWatt hour (kWh) is a common unit of electrical output. 1 kWh means 1000 Watts used for 1 hour, say a 1 bar electric fire for 1 hour. It is often the unit of electricity pricing e.g. 1 unit on your electricity bill = 1 kWh.
This is commonly used for very large or small numbers. For example, 1 billion tonnes is the same as 1 gigatonne (1 Gt), or 1 x 109 tonnes. 1 millimeter (1 mm) is 1 x 10-3 meters (0.001m), etc.