The Padrón (empadronamiento).
It is obligatory by Spanish law to register on the padrón at the Town Hall in the area in which you reside. By registering on the padrón, you will actually be benefiting your local community. The fact is that your local government receives funding depending on how many people are registered on the Padrón. The Padrón is a good way for the Town Hall to know how many people are living in their area. Listed on the Padrón should be the full names of all persons that are living at the property. You can still be on the Padrón for a certain address even though your name is not on the escritura or on the rental agreement.
If you wish to apply for your certificado, register your child for school, register at your local medico (doctor), then you will likely need a padrón.
How to obtain my Padrón?
You need to go to your local area Town Hall. You should take with you your passport, a utility bill, a copy of your escritura and your NIE number (Numero de identidad de Extranjero). Signing on the padrón is normally a fairly quick process and you will be given your certificate to take away with you.