DNS settings

Well DNS=Domain Name System and here is the wiki for DNS that would be a good reference.

IMHO there is a DNS bug in 10.6 discussed in this blog. To get to the settings open System Preferences->Network pane. In the Network pane hit the "Advanced" button to get a drop down. If you are using a router that is more than a year old then in the TCP/IP tab uncheck the IPv6 box and save, since the router does not use IPv6. Also There is the DNS tab where one can put a known good DNS server in.. I personally use OpenDNS.com and have a free account discussed in this instructional video to block know Trojan & **** hosting sites. It actually sped up my surfing but your mileage might vary. I use their service to block because they use the trojan/phish reporting site PhishTank.Cm for their reference to block.

If it is on an individual Mac that connects to a router, the DNS settings should automatically have the router's address and no search domain.

If it is the router connected to a modem, then the DNS settings should be those of the ISP, set automatically. They show up grayed out in the Airport Utility. You can just type over them.

You can use another DNS if you want. Google has one, but I don't know their addresses. I use OpenDNS which are 208.67.222.222, 208.67.220.220. If you set those in your router, then all connected computers will use those search domains. If you set them on your computer, you would got to Network System Preferences, Click on the Service you are using to connect (Airport, Ethernet, etc) and Click on Advanced and go to DNS tab.

The plists are in your HD/Library/Preferences/SystemConfiguration/ folder.
Getting rid of all of these will pretty much wipe the whole network setup clean:
com.apple.airport.preferences.plist
com.apple.nat.plist
com.apple.network.identification.plist
NetworkInterfaces.plist
preferences.plist

You'll have to add the Services back into Network Preferences. Just click the Add ( + ) button and pick from the available Services.