DDoS

In a denial of service attack an attacker floods a domain name or IP with millions of hits a minute or a second, effectively taking whatever is hosted a that locale offline. If you are trying to visit a website while it is also getting a million hits, the web site likely will not respond as it cannot handle the requests and your request does not become, most likely, one of the few winners to get a result in lieu of waiting for a result. This sort of attack is not hard to counter as the hosting provider may just block the IP address that the attack is coming from, however in a distributed denial of service attack (DDoS), the millions of pings come from numerous and possibly ever-changing sources so that the effect may not be stopped as painlessly.