What is DDoS attack?

Hello all,
My friends just told me that he came across some security thing as DDoS attack. It is something related to IP and virus. What is it exactly and how to prevent it?

Thank you for any suggestion.

DDoS attack is Distributed Denial of Service, it is an attack where multiple compromised systems (which are usually infected with a Trojan) are used to target a single system causing a Denial of Service (DoS) attack. Many DoS attacks, such as the Ping of Death and Teardrop attacks, exploit limitations in the TCP/IP protocols. For all known DoS attacks, there are software fixes that system administrators can install to limit the damage caused by the attacks. But, like viruses, new DoS attacks are constantly being dreamed up by hackers.

DDoS attacks involve breaking into hundreds or thousands of machines all over the Internet. Then the attacker installs DDoS software on them, allowing them to control all these burgled machines to launch coordinated attacks on victim sites. These attacks typically exhaust bandwidth, router processing capacity, or network stack resources, breaking network connectivity. Only the best wat to prevent it to securing your server.

A denial-of-service attack (DoS attack) or distributed denial-of-service attack (DDoS attack) is an attempt to make a computer resource unavailable to its intended users. Perpetrators of DoS attacks typically target sites or services hosted on high-profile web servers such as banks, credit card payment gateways, and even root nameservers. To prevent the attacks one can use Firewalls, Routers, etc.

dotDefender is good software to prevent these kind of attacks. dotDefender is a software based Web Application Firewall. dotDefender complements the network firewall, IPS and other network-based Internet security products by intercepting seemingly legitimate users attempting to use the web applications to commit fraud or gain unauthorized access to valuable and confidential information.