Have you heard of the "padrón"?
The "padrón" is a municipal list of residents. A "certificado de empadronamiento" refers to a certificate the town hall will give you on request to demonstrate that you are registered on the padron.
The process to register is simple and does not require that you have your residency papers in order. It is free and does not incur any additional taxes. you will need to fill in the form, have your passport and some form of proof of the address shuch as escritura for home owner, rental contract for renter or utility bills. The registration should be for life or until you move house to another town and de-register ( "hacer la baja" ) Some foreign residents have reported receiving letters asking them to re-register after a few years.
It is important for all residents in a municipality to register in order for the local government to receive the proper amount of funds necessary to run the city and maintain infrastructures. Whether you are purchasing property or renting a home, it is in both your own and your community's best interest to make the little effort required to go down to the town hall and register.
More recently many town halls, as an encouragement, offer discounts on certain taxes like IBI for those on the padron and also certain services like courses.
This way, you will ensure that your sewage system continues to run smoothly. Rubbish will disappear on schedule and your local health clinic will be adequately staffed. All along the Costa del Sol, basic services suffer because so many foreign residents do not register for the "padrón" and therefore do not count when it comes time for the central government to allot budget to their local municipalities.
A "certificado de empadronamiento" can be requested for oneself, or perhaps for another on production of a identity document. The certificate is free in some places and up to 5€ in others, it is issued inmediately and is usually valid three months. It is requested as the proof of address by other government department including the residency office. There is also another certificate than can be requested called the "certificado historico de empadronamiento" (historical certificate) and includes more historical information about you registration. This may take a few days to produce.
The electoral role is a parallel register. You will probably need to request registration on this separately. This is the registration used in the elections. EU residents may vote in local and EU elections but not National nor Regional elections.