The components that make up the Internet are threefold, as illustrated in 1.1.
First there are Hosts, the workstations, PCs, servers, and mainframes that we run our applications on. Then there are networks, the Local Area Nets (Ethernets), point-to-point leased lines, dial-up (telephone, ISDN, X25) links, that carry traffic between one computer and another, and finally there are routers. These glue together all the different network technologies to provide a ubiquitous service to deliver packets (a packet is a small unit of data convenient for computers to bundle up data for sending and receiving). Routers are usually just special purpose computers which are good at talking to network links. Some people use general purpose machines as low performance (low cost) routers - e.g. PCs or Unix boxes with multiple LAN cards or Serial line cards or modems.
Figure 1.1: A Piece of the Internet