A BER is in effect an Energy rating for your home. It allows people to make like for like comparisons as to the energy efficiency of the dwelling. From January 1st 2009 a BER is compulsory by law for an residential home being sold or rented. All new houses must now come with a BER.
There are some exceptions for buildings that dont require a BER, such as some types of commercial building, religous buildings and farm structures.
Below you can see thermal images of two houses, the one on the left built to a very high A rating, the one on the right with a D rating. The difference is dramatic and so is the cost in heating these two respective homes.