"The fishing fleet lies dark against the sun-washed sea. Along the Tyrrhenian waterfront, a stressed football manager, unable to sleep, takes an early-morning walk. Oblivious to the shrieking of the gulls and the haggling of the dockside mongers, he strides on, asking himself again and again how he can get the best out of his side, ponders how he can strengthen a defence that, for all his best efforts, remains damagingly porous. As he paces the harbour, churning the problem over and over in his head, a boat catches his eye. The fishermen haul in one net, swollen with fish, and then behind it, another: the reserve net. This is his eureka moment. Some fish inevitably slip the first net, but they are caught by the second; he realises that what his side needs is a reserve defender operating behind the main defense to catch those forwards who slip through. That manager was Gipo Viani, his team was Salernitana, and his invention was catenaccio."

-Excerpt from Jonathan Wilson's Inverting the Pyramid: The History of Football Tactics on the development of catenaccio, the system made famous by Helenio Herrera at Inter Milan in the 1960s. The system employed a libero, or sweeper, who sat in behind a line of man marking defenders to provide cover and focused on counter attacking with long balls out of the back. In Herrera's version, the libero played behind four man marking defenders (in other versions the libero sat behind three defenders), creating a five man defense in a 5-3-2 formation. Inter would win three Italian league titles, two European Cups and two Intercontinental Cups under Herrera.

Herrera's version of catenaccio ultimately fell out of favor after Inter were beaten 2-0 by Ajax in the final of the 1972 European Cup. Inter's rigid four man markers were drug all over the field by the fluid movement of Ajax's total football and, subsequently, zonal defending became the norm in professional football.

While man marking is a thing of the past and no top level teams use a traditional libero, certain teams have employed systems that, if not directly influenced by catenaccio, have stemmed from the need to address the same concerns catenaccio was attempting to address: particularly the need to have a free man at the back to provide cover. Juventus's current 3-5-2 system is quite similar to Hererra's 1-4-3-2 catennacio. Both used three central defenders. Although Juventus's three central defenders are more fluid and there is no designated libero to sit deep, the idea is to allow two to pick up attackers moving into their zone while one can drop slightly deeper and provide cover, the same philosophy behind Herrera's system (although Herrera's two center backs marked men rather than zones). Three central defenders allow Juventus's full backs to push higher up the field into attacking positions when in possession, operating as what we call wing backs. Again, this is something Hererra was doing in the 1960s. With ample cover in the center of defense, he would allow left back Giacinto Facchetti license to push on and join in the attack.

Michael Cox has written in his Zonal Marking blog of teams using a more modern version of a sweeper whose positioning is different to that of the traditional deep lying sweeper. He suggests the modern version of a sweeper is a defensive center midfielder that plays in front of a four man defense with attacking outside backs. When his team is in possession, this holding midfielder drops into the center of defense while the two center backs move wide on either side of him, forming a back three. This gives the two outside backs the freedom to push into the attacking third without leaving only two center backs to defend any counters that may spring. Barcelona have used this system to great effect with Sergio Busquets dropping between center backs Carles Puyol and Gerard Pique, allowing Dani Alves and Erica Abidal (now Jordi Alba) to get into very attacking positions. I highly recommend this article from Cox if you want a more detailed explanation on the use of this more modern sweeper system. Jonathan Wilson also wrote a fascinating piece on this subject for the Guardian back in 2010 that you can see here.

Although both Barcelona and Juventus's systems represent modern day versions of sweeper systems, they're quite different in style and philosophy. The defense in Juventus's 3-5-2 sits deeps, invites the opposition to get forward, then looks to spring quick counters. Barcelona's defense makes the field compact by holding a very high line while the 6 attacking players press the ball. Their attack is focused on ball retention and dominating possession. Both employ three at the back with fullbacks providing width high up the field when in possession, but they differ in where on the field they like to do their defending and the importance placed on keeping the ball. These two teams offer a stark example of how systems fairly similar in defensive shape can be quite different in practice.