Spain uses the Euro. Credit and debit cards are accepted nearly everywhere. ATMs are widely available 24 hours a day, 365 days a year.
Spain by train is a great way to see the country with a network (largely operated by Renfe) that links all major towns and cities. The addition of the high-speed AVE trains has facilitated and sped up travel between Madrid and other major cities including Barcelona (just over 2 ½ hours!), Seville, Malaga and Valencia. Other train lines also service the rest of Spain. Reservations are required for long-distance train travel, and you can take advantage of cheaper fairs by pre-booking. If you plan to make more than one or two train trips, consider buying a railpass, whether an InterRail, Eurail or Spain pass. Renfe also offers several luxury train routes with 5-star accommodations and service.
Spanish motorways (autopistas) are some of the best in Europe thanks to a recent road-building revamp, but tolls apply and can get expensive. There are also non-toll roads (autovías), which can get a bit busy. Watch out for radar speed traps that carry steep fines which must be paid on the spot in cash to the arresting officer. Tip: when stopping for gas, lock your doors when going to pay.
Spain has a number of national airlines and charter companies. The most well known of course is Iberia, which flies throughout the country and internationally. The Spain-based Air Europa flies offers national and international flights with destinations in Asia, North and South America, the Caribbean, Europe and North Africa. Air Nostrum is a regional airline, and there are several smaller operations offering regional service.