Watts are a measure of power.
Power is energy per second.
Therefore, as an example your standard 1000W kettle uses 1000 Joules in a second. (Or 2000 Joules in two seconds or 500 Joules in half a second and so on.)
A Joule is a unit of energy (or 'work done') and is defined, IIRC, as the amount of work done to exert a force of one Newton through a distance of one metre. It can also be defined as the work needed to move an electric charge of one coulomb through through a potential difference of one volt.
The latter equates to a current of one amp going through a resistance of one ohm for one second.
You'll most commonly see Watts in regards to electrical items, but it is used for a massive variety of things, such as heat dissipation.
Is there a particular reason you need to know?
Hope that helped!