If you are an EU citizen you will only need your passport or ID card to travel to Spain.  If you are traveling with a child under age 16, you will need parental authorization with an Apostille, if both parents are not traveling with that child. Grandparents traveling with grandchildren must have authorization from the child’s (children’s) parents, with an Apostille, granting permission to travel with them. 

Most travelers will be allowed to stay in Spain for 90 days.  Check the conditions for travel with your local Spanish embassy to be sure of the duration of your visa. 

If you are from a Schengen country you can travel freely within the borders of all Schengen countries. 

Americans:  No visa is required for a visit up to 90 days.  You must have an American passport. 

British Citizens:  A British citizen is an EU Citizen.  Therefore, Brits do not need a visa and are free to move to, live and work in Spain without special permission. 

Canadians: No visa is required for a stay of up to 90 days.  A passport is required. 

Australians: No visa is required for a stay of up to 90 days.  A passport is required. 

For all the above citizens, you cannot enter Spain on a tourist visa and then decide to extend your stay.  You must return to your home country and apply for the appropriate visa from there. 

RESIDENCE VISA
If you think you want to live in Spain on a permanent basis, contact the Spanish consulate or embassy in your home country about the requirements before traveling to Spain, as you may have to travel home again to obtain the necessary documents and the correct visa.

A Residence Visa is not a Residence Permit. 

A Residence Visa is granted for one (1) year, and may be renewed for an additional two (2) year period.  After then, the renewal process happens every five (5) years. 

A Residence Visa must be applied for within your home country prior to travel to Spain.

RESIDENCE PERMIT FOR NON-EU FOREIGNERS
The Residence Permit can be applied for while in Spain by visiting the Foreigners' Office (Oficina de Extranjeros) or the National Police Station with a foreigners' department nearest the city or town where you are going to live. 

You must apply within fifteen (15) days of entering Spain, and you must be traveling on a Residence Visa, not a tourist visa.  The following documentation is required:
1.  Valid passport
2.  Residence Visa in force
3.  A long-term rental contract or receipts for rent
4.  Certificate of criminal record issued by the authorities of the foreigner's home country.
5.  Passport-size photographs
6.  Medical certificate (certificado medico)
7.  Proof of financial support, including pension, salary or Social Security
8.  Proof of health assistance
9.  Marriage or divorce certificate or other papers relating to your marital status, with a Spanish translation (Apostille)

There are different types of Residence Permits:
Temporary Residence Permit: allows you to remain in Spain for a period of time between 90 days and 5 years.  Your Residence Permit may be renewed.
 
Permanent Residency Permit: It is available to all foreigners who have held a normal Residence Permit for a continuous period of 5 years.  Renewals of residency permits are allowed, provided that neither personal nor economic situations of the non-EU foreigner have changed, and should be made at least a month before the Residence Permit expires.

You should carry your residence card with you at all times as it constitutes a mandatory identity card for foreign residents in Spain, and it shows your fiscal registration number (NIE), which must be presented before entering a purchase agreement on a property.