Football, or futbol (when in Spain), is far and away the sport of choice in Spain. The Spanish national team is consistently one of the top ranking teams in the world, and recently won the European Championship in Austria - which is widely considered the second most prestigious international championship behind the World Cup.
The professional league in Spain is known as La Liga, which, along with the English Premier League, and Italian Serie A, attract the most talented players in the world. Having said this it is worth noting that Spanish football tickets are significantly cheaper than both Premiership and Serie A football tickets. Top teams include Real Madrid and Barcelona, with the likes of Robben, Casillas, Messi, Henry, van der Vaart, Puyol, and Eto'o among the international superstars playing for these teams.
Spanish football teams are run quite differently than many teams in other countries football leagues. They are actually athletic clubs which have members. If the members are not happy with the direction of the club, they can choose to vote out the president. Many of the teams in La Liga have been around for 100+ years, having a long and storied history. While Real Madrid and Barcelona are nearly always at the top of the league, other teams must fight to reach the top, and also to avoid relegation.
When a team finishes in the bottom three in the standings, they are relegated to the lower class league, and the top teams from the lower league are promoted to the top league. This would be like a Major League Baseball team being demoted to Triple A after a bad season. As you can imagine, this provides constant excitement and tension not only at the top of the standings, but at the bottom too.
Fans in Spain are very passionate about their teams. When the game is being played, nothing else matters. Supporters of clubs sing and dance throughout games, showing their support and passion for their clubs. Loyalty is passed down trough generations. If you ever get the opportunity, be sure to head the Camp Nou (home of Barcelona), Estadio Santiago Bernabeu (home of Real Madrid), or any of the other storied teams and experience the true passion and talent of Spanish football.