A process of elimination is the best approach to solving network issues.
When you get broadband, your internet service provider (ISP) normally supplies you with a broadband modem. This, plus your Mac, is the simplest network you can have, and for lots of people, it’s all the network they need. If you have DSL broadband -- the most common kind -- it’s delivered to your house through the phone line. At the phone socket, you plug in a filter that splits the frequency range so that the lower 4KHz is used by voice phone calls and the rest is sent to the modem.
All current iMacs come with built-in Wi-Fi capability as well as a wired Ethernet port.
The modem takes the analog electrical signal from the phone line and extracts the digital data stream encoded within it. This data takes the form of network “packets” that are wrapped up using an addressing protocol called TCP/IP. (This is a fairly meaningless acronym, so don’t bother trying to remember what it stands for.) Each TCP/IP packet contains a small piece of a website, or a graphic, or a downloaded file, or whatever is being sent from the internet. And every packet has a number, called an IP address, which identifies which computer or device it’s intended for.
(Via Mac|Life all.)