There are two different types of monitor on the market.
The first one plugs into a normal socket and then your appliance plug fits into that, this will give you the individual usage/cost of each appliance.
Or you can also buy another wireless monitor which attaches to your incoming mains feed, (in your electricity cupboard), and sends information wirelessly to a monitor elsewhere, this tells you how much electricity you are using at any time, so you can see how much electricity you save by, for example, turning off your tv, rather than leaving it on stand by.
Both of these products are available from most major supermarkets.