Healthy rivalries exist in all sports and soccer is no exception. Be it the baseball teams of the United States, or local teams in Africa playing soccer rivalry exists everywhere. In fact such competition can excite followers and increase ticket sales for such events; one famous rivalry that has existed over the years is between FC Barcelona and arch-rivals Real Madrid. Their matches in the Spanish league are eagerly followed all around the world and are generally considered to be the largest crowd pullers worldwide.
FC Barcelona (also normally well-known as Bara through its fans) and Real Madrid were opponents long before they came into the soccer field and are the two largest clubs in Spain - Catalonia. When each of these clubs joined the Spanish football league the rivalry that resulted became legendary - check the Barcelona schedule to see when Real Madrid has visited the Nou Camp over the years.
After General Franco banned all peripheral 'languages' such as Catalan, this rivalry started to take on political overtones following the Spanish Civil War. Catalonia has at all times been independent in its outlook and required equality and was therefore anti Franco. While Real Madrid did not suffer such persecution, FC Barcelona came under the glare as a central point of Catalonian tradition; and Franco simply favored Real Madrid.
That extreme opposition continued in to the 1950s when each club searched for players and at times guides from both teams would find themselves in situations where they were trying to recruit the same player, such as Alfred Di Stefano. Some players would go away from Real Madrid and join FC Barcelona and vice versa. Fans did not like this and saw such players as being deserters.
At present the opposition between the teams is enjoyed by fans and players in equal measures and the trend continues. If you want to see "el clasico" at the Nou Camp make sure you buy FC Barcelona tickets online and appreciate first hand the magnitude of the occasion.