An Energy Performance Certificate (EPC) is a report on the energy efficiency of your home. The report contains graphs, as shown in the sample below, which give a current and potential energy rating.
As of the 1st October 2008 if you are letting a property to a new tenant you require an EPC by law. If you are a home owner concerned with the efficiency of your property then an EPC will give you a professional assessment with suggestions of how to make your property more energy efficient.

The efficiency rating is determined by criteria which the Domestic Energy Assossor (DEA) is trained to analyse. Some of the things they look at are listed here:
- The approximate year of construction
- Size of property
- Construction materials
- Glazing materials
- Heating system - Boiler, thermostats, controls etc
- Loft insulation
As well as the energy efficiency graphs the report also makes suggestions for how to improve the property's energy performance, which will lead to reduced carbon emissions, lower energy bills and make the house more attractive to prospective buyers or tenants.
For more information:
http://www.businesslink.gov.uk/bdotg/action/layer?topicId=1082185007
http://en.wikipedia.org/wiki/Energy_Performance_Certificate