It depends on the router and the distance. If the distance is less than 100 meters and the routers have copper ports (most do) you can connect them via copper. If the distance is more than 100 meters and has fiber ports, fiber cable is used to connect them.
Copper cable is usually named for UTP Cat cables. Other name is twisted pair cable.
Copper cable is the type cable that connects your computer to the wall jack in your LAN environment. This is if you are not using wireless.
As Reza mentioned it could depends of the distance and the type of port but if you are going to connect 2 routers directly one next the other through ethernet port you can use a crossover cable.
Hope it is useful
Haven't seen you for some time. How did your last Q&A go? Did you get good grades out of it?
Like any network device, connection can be done in several methods:
Today, it's often Ethernet cables of some type, but one of the features of routers is very general media support, so you may find all kinds of cables used, especially when dealing with "WAN" connections.
For example, in your later post, you mention "serial". That's often a cable to connect to something that works with a Telco's ("slower") WAN links.
Ethernet cable are specially designed for connecting two computers to each other or to connect router to router. Mostly Ethernet cables are designed to connect one computer to a router or switch. Ethernet cables are physically manufactured in two basic forms called solid and stranded.
An Ethernet cable is one of the most popular forms of network cable used on wired networks. Ethernet cables connect devices together within a local area network, like PCs, routers, and switches. To install, test, troubleshoot and certify ethernet cable you need specific ethernet cable testing tool.
Fluke Networks offers many cable testing tools for cable verification.