An electoral roll is a list of people who are eligible to vote in a particular place. They provide the person's name and address and sometimes additional information such as occupation.
Ancestry provides an electoral roll collection that is relatively new, and the British Library is expected to put a collection on line this year as well.
The "GenealogyInTime Magizine" has an interesting article on this topic and explains why this source will change the way people search for their UK ancestors. Check it out at Searching Electoral Rolls for Ancestors