Empadronamiento - is a municipal register that lists all the residents of a municipality, both those who live there permanently and those who are staying temporarily. The data is a proof that you live in a particular city. According to the law, every person living in Spain has a legal duty to register where they live.
Empadronamiento is a document that proves where and how long you are staying in Spain, regardless of nationality or legal situation, that is regardless of whether you have an address or not or whether you are in the process of completing all the necessary formalities. Getting inscribed on the list of the town's residents is a proof of registering.
To get registered you need a proof of residence. It doesn't have to be your own property it can be a a house or a flat you are renting or a house belonging to your friends or relatives.
It is advised to register in Spain straight after you arrive. Inscription in the registry can be done at the Town or City Hall of the town you live in.
Registering is a proof of setlling in the country and it allows you to apply for permanent residency. Another benefit that you receive from getting registered is getting access to social care offered by the Town Hall.
here is a legislation from 2003 according to which all EU citizens must renew their registration every two years if they don't have a permanent residence visa.