Basically, what this malware does is that whenever you type a web address into your browser, your computer contacts DNS servers to discover the numerical IP address of the site you are looking for and then takes you there. The DNSChanger malware messes with an infected computer's settings and directs it to a rogue server set up by an internet crime ring.
According to an Attorney General for the United States in an FBI press release, these criminals "were intentional cyberbandits who hijacked millions of computers at will and rerouted them to Internet Web sites and advertisements of their own choosing -- collecting millions in undeserved commissions for all the hijacked computer clicks and Internet ads they fraudulently engineered."
However, the FBI busted up the crime ring last year and took control of the rogue servers. Considering so many infected computers relied on the servers to reach the internet, the agency decided not to shut them off and instead converted them to legitimate DNS servers. However, running these servers is costing the government money, which is why they are being turned off in July. If you are one of the unfortunate ones whose computer got infected, the internet will no longer exist for you.
The DNSChanger Working Group (DCWG) has created a website designed to help you diagnose your computer and see if you are infected and remove the DNSChanger if necessary. I'd recommend checking it out, especially before July 8, or you can say goodbye to Facebook, Google and whatever else you use the internet for.
Source: CNET - Web could vanish for hordes of people in July, FBI warns