Ethernet (Cat 5, 6, etc) consists of multiple pairs of twisted low voltage wires that can be used to carry computer data or telephone signals. Questions should limit themselves to running and maintaining the wires only. General questions about networking should be directed to http://superuser.com
Ethernet is typically made up of four pairs of twisted low-voltage wires. For computer usage, typically only two pairs are actually used. Some Power over Ethernet(PoE) applications(mainly powering remote WiFi devices) will use all four pairs. Ethernet has a maximum distance of 100m/328ft before the signal degrades and a repeater needs to be installed. Ethernet is sold in Riser(standard runs where you would normally find other wires) and Plenum(runs that use an air duct for part of the run).
Ethernet can also be used for phone wiring, with up to four phones sharing a single Ethernet. The wires are the same gauge as those found in Cat3(phone). Indeed, the RJ45 Ethernet jack can accept a RJ11 phone plug.
If you have questions about planning and setting up a computer network, or networking equipment, please visit Super User, the Stack Exchange site for general computing questions.