What is El Clasico ?

El Clásico (Spanish pronunciation: [el ˈklasiko]; Catalan: El Clàssic, pronounced: [əɫ ˈkɫasik]; "The Classic"), is the name given in football to any match between Real Madrid and FC Barcelona. Originally it referred only to those competitions held in the Spanish championship, however, 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 89 wins. Barcelona leads the count in official titles won with 79 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.

source : Wikipedia