When you get your electricity bill you will see that you are charged for the number of units you use.
A unit represents 1 kw (kilowatt) hour . This is 1000 watts for on hour.
So, for instance, if you have 10 lights bulbs on for one hour which are all 100 watt bulbs this will use one unit.
Alternatively something like a 2 kw fan heater (that's 2000watts) on for only half an hour will also use one unit.
All appliances should tell you what wattage they are rated at so you could(if you wanted to) calculate how much they cost you to run.