Server cluster (known also as server farm) - is a bunch of computer servers that are usually managed by one of the companies to get the performance and results that otherwise cannot be achieved by using of the single computer. Server clusters often consist from thoustands of computers and there is a great need of power for them to operate and cool.
Server clusters usually have separate backup servers. Such servers can stand for primary servers in case they fail to work correctly. Server clusters are usually located in the same place as network switches or routers that ensure the connection between different parts of cluster and between users of clusters. Usually such servers, as well as routers, power supply and other related electronics is positioned in 19-inch racks in server room or data-center.
Usually server clusters are used for cluster computing. Many modern supercomputers are comprised of giant server cluster with high-speed processors that are linked to each other by using Gigabit Ethernet or other solutions such as Myrinet or Infiniband. Also server clusters are often used for hosting purposes (they are called web-clusters), scientific modelling and rendering.