A denial of service attack (DOS) is an attack through which a person can render a system unusable or significantly slow down the system for legitimate users by overloading the resources, so that no one can access it.
If an attacker is unable to gain access to a machine, the attacker most probably will just crash the machine to accomplish a denial of service attack,this one of the most used method for website hacking

Types of denial of service attacks

There are several general categories of DoS attacks.Popularly, the attacks are divided into three classes:

bandwidth attacks,
protocol attacks
logic attacks

for further Information see article on DOS on WIkipeedia.

What is DDos? And how dangerous is it.

To hack a website An attacker launches the attack using several machines. In this case, an attacker breaks into several machines, or coordinates with several zombies to launch an attack against a target or network at the same time.
This makes it difficult to detect because attacks originate from several IP addresses.
If a single IP address is attacking a company, it can block that address at its firewall. If it is 30000 this is extremely difficult.

You can imagine that is way shut down the Yahoo website in year 2000 for 3 hours.(It was most used website at that time)