Internet hosts are connected by all kinds of communication links over which data is exchanged. Any communications link will suffice, so long as it can transport IP packets. Internet links have been operated using amateur radio transceivers, between high-voltage experimental physics apparatus, and over parallel printer ports. The higher level TCP/IP protocols strive to make no assumptions about hardware details.
However, there is clearly a need for standardization for communication over common hardware technologies, such as Ethernet. So, while not strictly part of the Internet protocol suite, many RFCs document standard ways to transport IP packets over various common hardware types. RFC 2400 §6.3 has a list of these Network-Specific Standard Protocols. Ethernet and serial lines are sampled below; others such as FDDI and Token Ring are not discussed here.
Computers interconnecting communications links together are refered to as intermediate systems because they relay traffic between other hosts. They include routers, bridges, and switches. The distinctions is not always clear. A router allowing an network administrator to login via telnet behaves as an end system. An end system with multiple network connections can act as a router. The difference between a router and a bridge is only one of software; systems capable of acting as both as sometimes called brouters. Cisco Systems manufactures a Route Switch Processor (RSP) which plugs into a switch and allows it to act as a router, bridge, and switch, all more or less simultaneously.