DNS stands for Domain Name Server. Basically what it does is convert a hostname like google.com or phrack.org into an IP address like 127.0.0.1. Well, it's a bit more complicated than that, but that's the main idea. Right now you're probably using your ISP's DNS servers. This can be a problem if you're from a country like Egypt or China. Check out this article on DNS cache poisoning: http://viewdns.info/research/dns-cache-poisoning-in-the-peoples-republic-of-china/.
DNS cache poisoning is when a device along the path of a returning response of a DNS query is changed into something else. Let's say you typed in your browser google.com, and instead of giving you one of the correct IPs like 74.125.230.230 it gave you 66.29.212.73. Your ISP does this in a way too..check out the link.