DDoS attack though used criminally but is an intelligent technology. It has evolved with time and now follows the domain name and the IP address it points to. It is somehow a layered threat and hinders the access to the site in following different ways
Vulnerable Zoombies Attacks: A DDoS attack directs hundreds or even thousands of compromised "zombie" hosts against a single target. These zombies are unintentionally recruited from the millions of unprotected computers accessing the Internet through high-bandwidth or always-on connections. By planting "sleeper" codes on these machines, hackers can quickly build a legion of zombies, all waiting for the command to launch a DDoS attack. With enough zombie hosts participating, the volume of an attack can be astounding. DDoS Threats Over the last few years, attackers have refined their methods. As developers make software more reliable and more resilient to DoS, the attack vectors have changed to target hard-to-secure parts of a service.
Bandwidth and Traffic Bottlenecks: A server that has a bandwidth of 100 Mbit/s can easily be attacked by a DoS attack with a machine that has a little more bandwidth than this which will make it loss packets and finally will make it unusable. This creates bottle necks for other site surfers and will make the server itself dormant.
SYN Attacks: SYN (synchronous) stacks are becoming frequent nowadays. This attack can prevent access to your mailbox, WWW and other critical servers. The SYN attack (sometimes referred as SYN flood attack) sends TCP connections requests faster than a machine can process them. The SYN attacker creates a random source address for each packet and sets the SYN flag in each packet. Victim responds to spoofed IP address, then waits for confirmation that never arrives. Victim's connection table fills up waiting for replies. After table fills up, all new connections and legitimate users are ignored. Once attacker stops flooding server, it usually goes back to normal state (SYN floods rarely crash servers).
Making Applications Flawed: DDoS attacks do not only attack over the network layer but the their inscrutable nature is that they even make the applications malfunctioning. This might also be achieved by deploying load-intensive WebPages