Networking and communication equipment are absolutely necessary in a data center to maintain a high-bandwidth network for communication with the outside world, and between the servers and other equipment within the data center. This includes components like routers, switches, the servers' network interface controllers (NICs) and potentially miles and miles of cabling. Cabling comes in various forms including twisted pair (copper), coaxial (also copper) and fiber optic (glass or plastic). The types of cable, and their various subtypes, will affect the speed at which information flows through the data center.
All that wiring also has to be organized. It's either run overhead on trays hung from the ceiling or attached to the tops of racks, or run underneath a raised floor, sometimes on under-floor trays. Color coding and meticulous labeling are used to identify the various wiring lines. Raised floors of data centers generally have panels or tiles that can be lifted for access to get to cabling and other equipment. Cooling units and power equipment are sometimes also housed below the floor.
Other important data center equipment includes storage devices (such as hard disk drives, solid state drives and robotic tape drives), uninterruptible power supplies (UPSs), backup batteries, backup generators and other power related equipment.
Data centers also have lots of equipment to handle temperature and air quality control, although the methods and types of equipment vary from site to site. They can include fans, air handlers, filters, sensors, computer room air conditioners (CRACs), chillers, water pipes and water tanks. Some sites will also put up plastic or metal barriers or use things like chimney server cabinets to control the flow of hot and cold air to keep computing equipment from overheating.
And of course, software is needed to run all this hardware, including the various operating systems and applications running on the servers, clustering framework software such as Google's MapReduce or Hadoop to allow work to be distributed over hundreds or more machines, Internet sockets programs to control networking, system monitoring applications and virtualization software like VMware to help cut down on the number of physical servers.