What is a denial-of-service attack? Most commonly, these events occur when mischief makers or hackers simply flood a target computer with more traffic than it was built to handle. This can involve sending more connection requests than a server can handle, or sending enormous amounts of random data to use up the target’s bandwidth. They all essentially leave the victimized computer in the position of someone trying to sip from a fire hose.
Where does the traffic come from? Some attacks link a number of systems together to pour traffic into the target computer until it can't handle that data and do its regular work, too. In a distributed denial-of-service attack, the hacker uses a security flaw to seize control of other computers and use them to send vast amounts of data to a website or send spam to targeted email address. This is a distributed attack because the attacker is using many computers to create a denial-of-service.
Are these problems always deliberate attacks? No, there can be accidental denial-of-service events in which a web site has a sudden spike in popularity which produces more hits than it can handle. This can happen when a very popular website posts a link to a smaller, less robust site, crashing it.
Do these attacks steal data? Normally, the goal is simply to shut down the target, paralyzing an email system, for example. But they can cause serious damage to systems.