Many new arrivals are surprised to learn that more than one language is actually spoken in Spain. Castilian Spanish (known as Castellano) is the official language and is understood everywhere. However, you might not always be replied to in the same language!
One of our readers commented, "I spent five years learning Spanish, went to Barcelona and suddenly found out the Catalans are much keener on speaking their own language!". This sounds frustrating, but Catalunya is the most complicated case as people there are very proud of their language. The good news is that foreigners get a lot more concessions and you will probably be spoken to in Castellano once they realise. In cities you will generally always be OK speaking Castellano, but in small villages, especially with older people, things might not be so easy.
There are several regional languages/dialects in Spain (and not all are listed here), they range from very close to Castellano to completely different:
During the time of Franco's dictatorship, which ended in 1975, all regional languages were suppressed along with separate regional identities. But in modern Spain they have become increasingly important, and many now have the status of official languages alongside Castilian Spanish.
Knowledge of these languages/dialects can be useful, particularly if you are doing business in Catalunya. However, you will do just fine speaking only Castellano as it is universally understood. For the sake of simplicity, we will refer to Castellano as just Spanish going forward.