A denial of service or DoS attack is a way of overwhelming your servers so that they can’t cope and are effectively stopped from working. Web servers, mail servers, name servers, and in fact any kind of networked computer system can be taken off-line with this kind of attack.
It can be set in motion from one machine, but then it will typically use a large number of others to carry out its business. Because the majority of servers have security software like firewalls, it’s easy enough to shut the door on individual systems, but with a distributed denial of service (DDoS) attack, there can be literally hordes of systems (known as a botnet) all coordinated to simultaneously bombard a server with requests. With so many coming all at once the server is swamped and it grinds to a halt.
Hackers frequently use DoS attacks to try and cripple websites. You don’t need any authentication to flood a server with requests which means that even the most secure server can be vulnerable. One system alone won’t normally be able to achieve success with a DoS attack, hence the popularity of DDoS attacks.
Right now, there are plenty of security systems capable of detecting DoS attacks, but it’s a never-ending arms race, so you can never expect to be protected by any security provider for long.