1. The amp, short for ampere, is the amount of electric current flowing in a wire.

The electric current is measured in electrons per second.

2. The ohm is a measure of resistance to current.

The higher the resistance at a given voltage, the less current flows. Higher resistance can be obtained by using thinner wires or by using materials which are less efficient conductors.

3. The volt is a measure of the potential energy of the source or the "pressure" pushing electrons through the wire.
4. A watt is a unit of electric power or the speed at which an electric current can function.

Electric energy is expressed in units called watt-hours, which is what is usually referred to in everyday speech when someone says "watts".

A 100-watt light bulb, that operates for one hour, uses one watt-hour and electricity which is used in a home is normally metered in kilowatt-hours, or thousands of watt-hours.