NIE-number: La Caza arranges for your foreign tax number in Spain (NIE-number).
Just like you have been accredited with a Social Security Number in your home country, so should every foreigner who is in any way liable to pay tax in Spain have a Spanish tax number, the NIE (Número de Identification de Extranjeros).
The NIE number is a tax identification number. You will need this number with every major transaction. Without NIE, you cannot buy or rent a house, buy a car, buy furniture, you cannot do major purchases, you cannot register with a municipality.
In short, you need this number in Spain for all kinds of matters.