Looking for What is a DDoS??
There are many effective methods to prevent or mitigate DDoS attacks. This article will cover a few suggestions.
No method can be 100% effective in preventing DDoS attacks.
Enabling security related modules/plugins such as mod_security and mod_evasive can be extremely beneficial when preventing DDoS attacks.
Some services such as CloudFlare provide a proxy-type of protection. It acts as a middle-man to determine whether or not your "visit" should be served the content.