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What Exactly Happened to Knock So Many Sites Offline?

Major Sites Suffer Outages: So What Is a DDoS Attack? 0:42

The cyber attack that brought down some major websites Friday is the "difference between taking a drink of water and being waterboarded," one cyber security expert told NBC News.

"When criminals manipulate web traffic to point toward a particular site in a short period of time, it overwhelms it," said Robert Siciliano, CEO of IDTheftSecurity.

Related: Massive Cyberattack Knocks Out Major East Coast Websites

What kind of attack was it?

Friday's attack was a DDoS, or distributed denial of service, and brought down a number of popular websites including Amazon, Twitter and Spotify.

How does it work?

A DDoS is typically pulled off with networks of "zombies," which are computers compromised by attackers that can be used to flood a server, Siciliano said.

Who was behind it?

It's unclear who orchestrated the attack and whether it could possibly be state sponsored. Siciliano said pulling off this level of attack takes some cyber savvy.

"Not everybody can perform a DDoS attack, but you could actually pay someone to do it, so anyone can with the right resources," he said.