Realtor.com's website has been down since midday Tuesday (update: the site is back online), along with other Move services like Top Producer. In a statement, the company identified a Distributed Denial of Service (DDoS) attack as the reason for the outage.

The attack against Move's web properties is the latest in a string of recent DDoS attacks across the Internet. Ancestry.com was also down Wednesday. The World Cup website, along with RSS reader Feedly and Realtor-favorite Evernote were targets of DDoS attacks last week.

Attack of the Zombie Computers!

So what is a DDoS attack, anyway? It's a tactic that malicious hackers use to take down a website or network by overwhelming it with a high volume of traffic. Think of it as a mass stampede of thousands or millions of computers all trying to access a site at the same time.

It's called a Distributed Denial of Service attack because the intent is to deny legitimate users from accessing a website, therefore crippling a business for the duration of the attack. Banks, media, and other major sites are frequent targets of DDoS takedowns.

Hackers use something called a "botnet" to execute DDoS attacks. A botnet is a network of linked computers that are used collectively to pummel a target's server. These computers often belong to regular home or business users, but have been infected by a virus or malware. Once infected, hackers can control these computers remotely and use them to attack a website.