As more people use more computers at home and at work, they want to be able to use files, printers, internet access, etc. from more than one computer. The answer is interconnecting the computers with network wiring. You can share as much or as little as you want. Perhaps you just want to use the other computer's printer, or use the files when you're using a laptop. More common now is the request to share internet access. Since Broadband (Cable Modem or DSL) can cost $40-$50 a month, per modem, if you have more than one computer, you want to avoid having more than 1 monthly fee. By the way, you don't have to leave one computer on for the other computer use the cable/DSL modem.
Some people opt to use wireless networking. There are issues to be understood before going that route:
Security - the modems have to be configured to prevent outsiders from getting access to your network, or your passwords, credit card info, etc. The default settings do not provide security.
Reliability - the wireless networks have evolved to different frequencies: 900 MHz, 2 GHz, 5 GHz. These same frequencies are used for your cordless telephones. When you use the phones, they can disrupt the networks. Even microwave ovens can interfere with the network functions. We have customers who tried wireless, but switched to hardwire due to these issues.
Distance/speed - some installations just don't provide the speed you need due to the distances between the network points.
For going with a hardwired computer network, here's the typical recipe:
Start with (1) Internet access with either a cable or DSL modem. ($40-$50 per month)
Add (1) router to control/manage access among the computers. (Purchase for about $100)
Add network cabling (also called CAT 5 or category 5 cabling) to link the computers together at the router. The cost of the wiring depends on the location of the computers and the access for running the wires. For 2 computers, the cost is usually $300 -$400. Connecting additional computers would cost about $250 each. The cost varies based on the access for running the wires and whether they are on different floors.