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google/wikipedia are your friends
http://en.wikipedia.org/wiki/Defender_(football)
Sweeper/Libero
The sweeper is a more versatile type of centre back that "sweeps up" the ball if the opponent manages to breach the defensive line. His or her position is rather more fluid than other defenders who man-mark their designated opponents. Because of this, the position is often referred to as libero (from the Italian word meaning free). The sweeper's ability to read the game is even more vital than for a centre back. A sweeper is sometimes expected to build counter-attacking moves, and as such requires better ball control and passing ability than a typical centre back. However, sweepers are often merely defensive players. For example, the catenaccio system of play, used in Italian football in the 1960s, employed a purely defensive sweeper who only 'roamed' around the backline.
Franz Beckenbauer, Gaetano Scirea, Franco Baresi, Frank Rijkaard are some of the best ever sweepers in football history. Paolo Montero and Rio Ferdinand would probably serve as the best modern-day examples. This is attributed to their elegant style of play, refined technique and ability to launch counter-attacks. Their defending styles are also considered more 'classy' compared to the modern day centre-back. For instance, Marquez is known for his ability to read and anticipate the attackers' moves. This allows him to intercept passes and get the ball off the opposition without needing to fling himself into tackles.
Some observers consider the position of the attack-minded sweeper to have evolved into the position of the defensive midfielder / deep-lying playmaker in modern football (see Midfielder (football)). Some of these 'evolved sweepers' include Andrea Pirlo, Xabi Alonso, Lee Carsley and Michael Carrick.