What is 100 watts?
Watts is a unit of power that is equal to volts x amps x Power Factor.
The watt is the SI unit for power.
Power is defined as 'the rate of doing work'. Since work is measured in joules, the watt is equivalent to (a special name given to) a joule per second.
The watt is sometimes described as measuring 'electrical' power. In fact, there is no such thing as 'electrical' or 'mechanical' (etc.) power, as power is simply a rate.