DoS/DDos Attacks - Explained

DoS is defined as "denial of service attack" or distributed denial of service attack(shortly DDos)" which is an attempt made by a person or a group of people to make computer resource unavailable to intended legitimate online users. Where as in distributed denial-of-service, large numbers of compromised systems which are sometimes called as a "botnet" attack a single target
(almost both are same :D).

There are some general ways by which they can compromise a system and proceed with their DoS attacks..

Excess Traffic:
In this type of attack hackers send more/massive traffic to a website which it can't handle so that it may crash and it will not be available to legitimate users. Unless the website has the capacity to handle such occurrences, the security is breached and it may take a while to resolve the damage. A site’s bandwidth or disk space simply cannot accommodate such numerous requests simultaneously.

Peer To Peer Sharing:
When peer-to-peer file sharing is done, the hacker can direct all peer-to-peer viewers to the target website. When thousands of online users connect to the target website, the tremendous amount of connections can harm your server, computer, and your online resources.

Nuke:
Nuke is an old type of attack, in this the attacker will send invalid/corrupted ICMP packets using a modified ping utility, so that the affected system will slow down until it stops.

Teardrop attacks:
In these type of attacks hacker will send mangled IP fragments with overlapping. This can crash various operating systems due to a bug in their TCP/IP fragmentation re-assembly code.

There are many other types of attacks like Permanent denial-of-service attacks, Application level floods, Distributed attack, Reflected attack, Degradation-of-service attacks, ICMP flood.

Finally, United Kingdom specifically outlawed denial-of-service attacks and set a maximum penalty of 10 years in prison.
In the US, they can be a serious federal crime under the National Information Infrastructure Protection Act of 1996 with penalties that include years of imprisonment. Many other countries have similar laws.

From: http://www.hoverpchacks.com/2010/12/dos-attacks.html#ixzz1F0scG5Ti