Basically DNS tells your computer where to find the website it is looking for. This is usually done by your ISP. You may not know though that there are better alternatives. I will mention three of the most popular here, Clearcloud, Google Public DNS and, my personal favourite, OpenDNS. All of which are free to use.
Firstly, you may be asking why you might need to use a DNS service. One reason, without going into too much detail, is speed. The three DNS services I have mentioned have improved DNS lookup times as well as setting their services up to adequately handle the traffic from clients. This is done in many ways that there is not the room to go into here but includes load balancing, prefetching and securing the servers.
The second reason, and the reason I changed my DNS from my ISP, is that security holes tend to be patched quicker with the three services above than the DNS servers of some ISP's do. When somebody finds a security hole in DNS then can re-direct your requests wherever they like.
OpenDNS blocks phishing attacks using Phistank. The three also help prevent denial-of-service attacks, malware and other spoofing attempts. Clearcloud checks the website against a known blacklist before it delivers it to you, and seems to lead to no speed loss.
Another major benefit which some people don't realise is that some DNS services such as OpenDNS add content filters. So if you had children you could prevent them from seeing adult material using the DNS service. Or just specify your own level of filtering.
OpenDNS also has the added benefit of fixing typos, such as youtub.com when you meant youtube.com.
So how do you do it? Pretty simple really. Open the properties of your network adapter (Network Connections > Local Area Connection Properties > Internet Protocol Version 4).
Clearcloud: 184.108.40.206 and 220.127.116.11
Google: 18.104.22.168 and 22.214.171.124
OpenDNS: 126.96.36.199 and 188.8.131.52
Then restart the PC.