(CNN) It is a rivalry like no other, a match which permeates beyond the white lines of the football pitch. It's a complex cocktail of sport and politics. It's the biggest game on earth. On Saturday, millions will watch arch enemies Real Madrid and Barcelona -- two of the most successful clubs in history, boasting two of the world's greatest players -- battle it out in the 236th edition of the game dubbed El Clasico. Trailing their great rivals by 11 points in the league, Real -- who last week won the Club World Cup -- cannot afford to slip up at home if they are to retain hopes of winning the Spanish league title.