El Clásico, is the name given in football to any match between Real Madrid Club de Fútbol (Real Madrid) and Futbol Club Barcelona (Barcelona). Originally it referred only to those competitions held in the Spanish championship, but nowadays – in order to satisfy marketing needs – the term has been generalized, and tends to include every single match between the two clubs: UEFA Champions League, Copa del Rey, etc. Other than the UEFA Champions League Final, it is the most followed club football match in the world, watched by hundreds of millions of people.

The rivalry comes about as Madrid and Barcelona are the two largest cities in Spain, and the two clubs are among the richest and most successful football clubs in the world. Real Madrid leads the head to head results in competitive matches with 90 wins to Barcelona’s 86. Barcelona leads the count in official titles won with 81 trophies (the three Inter-Cities Fairs Cup’s won by Barcelona are included since the tournament is the predecessor of the UEFA Cup and recognized by FIFA), while Real Madrid has won 76 trophies. Along with Athletic Bilbao, they are the only clubs in La Liga to have never been relegated. They are sometimes identified with opposing political positions, with Real Madrid viewed as representing Spanish nationalism and Barcelona viewed as representing Catalanism. The rivalry is regarded as one of the biggest in world football.
The rivalry was intensified during the 1950s when the clubs disputed the signing of Alfredo Di Stéfano. Di Stéfano had impressed both Barcelona and Real Madrid whilst playing for Club Deportivo Los Millonarios in Bogotá, Colombia, during a players’ strike in his native Argentina.[26] Both Real Madrid and Barcelona attempted to sign him and, due to confusion that emerged from Di Stéfano moving to Millonarios from Club Atlético River Plate following the strike, both clubs claimed to own his registration. After intervention from FIFA representative Muñoz Calero, it was decided that both Barcelona and Real Madrid had to share the player in alternate seasons. Barcelona’s Franco-imposed president backed down after a few appearances, as Barcelona’s side claimed, but Real Madrid say Barcelona’s decision was voluntary and Di Stéfano moved definitively to Real Madrid.
(Source: wiki – http://en.wikipedia.org/wiki/El_Cl%C3%A1sico)
This is the list of top 10 facts about El Clasico.