Most people have never even heard of DDoS attacks, which in plain language is distributed denial of service. This is a type of attack that started around five years ago, but good news is that there are several ways to prevent it from happening to you. It is predominantly larger websites that are more under threat from attack; however step one in preventing it is to realize that smaller sites might also be at risk.
Distributed denial of service used to be simple with an anonymous person starting a ping command from their destination, aim it at a targeted IP address, and running at full speed, flood the target address with ping packets or ICMP Echo Requests. This has hanged in the interim simply because attackers need higher bandwidth than the place they are targeting. Therefore, they moved to large hosts and started compromising servers at research centers or universities, as these places has large servers. This is from locations such as these that they launched their attacks.
Technology advancement unfortunately now allow them to launch attacks from botnets, as it is less apparent. It evolved again and currently there are numerous ways to launch an attack, however it is also possible to prevent them. Apart from realizing that you could be at risk make sure that your DNS is protected with equal amount of load balancing as all your resources and your web.
If you manage your network and serving your own data, you need to protect it at network layer. Ensure your firewalls are good; block things such as ICMP, drop junk packets and everything else at your network border. Use caching servers that provide loads of static content and in case of an attack make sure that you could quickly replace all dynamic resources with static. You also get DDoS detectors that will alert you when an attack starts therefore will you be able to deal with it immediately.