Generally, the providers of low-end DSL/cable modem service provide setup instructions for only a single computer and will provide support for only a single computer connected directly to the modem. If you have any other configuration, their setup instructions won't work and they may not provide support in the event of problems.
Terminology
IP Address: Four number between 0-255, separated by periods. Every computer connected to the Internet must have a unique address -- no two the same.
Firewall: May be a separate, hardware appliance or software on your computer. It keeps unwanted intruders from using the Internet to gain access to your computer.
Router: A hardware firewall that also allows more than one computer to share an IP address. The router actually has the Internet IP address and provides private IP addresses for those behind the firewall. There are large-scale routers that don't meet this definition, but it works for home/small-business routers.
Intranet: Your local network. It may or may not be connected through a router to the Internet.
PPPoE: Point-to-Point Protocol over Ethernet. It is an ugly way for ISP's to provide changeable IP addresses to users.
DHCP: Dynamic Host Configuration Protocol. Works more reliably than PPPoE, but a changeable IP address can still be a problem.
Static IP: Your ISP gives you a real, full-time, fixed IP address which never changes. This is, by far, the most desirable Internet connection.
