What is Distributed Denial of Service (DDoS) Attack?

Denial of service (DoS) attacks are attacks that are deliberate attacks on your network properties to deny service to legitimate users. When these attacks seemingly come from distributed sources, they become distributed denial of service (DDoS) attacks.

Few years back, it was common to use spoofing techniques where a hacker would actually use very few machines (or just one machine) and spoof mutliple IP addresses. To the attacked destination it would seem that the attack is coming from multiple IP addresses. However in the recent times, with the advent of infected PCs, increasing number of smart mobile phones, many botnets are available around the world, which can be used to launch a real DDoS attack.

For a detailed discussion, please visit:
Distributed Denial of Service Attacks - A Primer (Tutorial on DDoS Attacks)