Most people use the default DNS settings provided by their ISP, and while they are usually sufficient for most purposes, there are plenty of free options out there, like OpenDNS and Google DNS. Namebench is a free app that checks to see whether your current settings are optimized and, if not, which free option is best for you. Here's how to use it: Download and install Namebench. Fire it up and choose your settings. Keep the top two boxes checked. If you're concerned about network censorship, check the third box, and if you want to help the developers, check the last box. You can tweak the rest if you're outside U.S. or want to experiment with different browsers.
Click Start Benchmark and wait while Namebench runs its tests. It should take several minutes. A browser tab should pop open when Namebench is done and give you a list of DNS servers and how much faster they are than the one you're currently using, unless yours are already the fastest possible. Namebench does not change your settings, but it's generally pretty easy to do it yourself. Check with the instructions you got from your ISP to set up your modem and/or router and just substitute the DNS addresses you received from Namebench for the addresses given by your ISP. It's best to do this with your router, as it will assign that DNS address for all the devices attached to it. That's it! This can dramatically improve your browsing speed, and it's fairly easy to work through.