What is an NIE number?
The NIE number is an identification number for foreigners in Spain. NIE stands for Número de Identificación de Extranjeros. It is similar to a Spanish identification number (DNI) and you use it the same way. You will need an NIE number for almost everything you want to do in Spain: such as buying a car or house, opening a bank account or taking out a telephone subscription. Even the parcel delivery man will ask for your NIE number. So its use is a lot broader than you are used to with the Dutch BSN number.
You can request the NIE number from the National Police or with the help of a gestor or lawyer. If you are going to buy a house and you do not have a NIE number yet, usually your lawyer will arrange this for you.
The NIE number is given to you on a white A4 piece of paper. This document does not make you a resident. When you become a resident you will get a smaller green card in credit card format with your NIE number on it. The document with your NIE number or the green card are not proof of identity. There is no photo or nationality on it. For official acts you should always bring your European identity card or passport and your NIE number.