Denial Of Service Attacks
A denial-of-service (DOS) attack is an attempt to make a web service or website unavailable by flooding it with network packets.
Online applications tend to have a limited bandwidth, so if an attacker can tie up an application with huge amounts of malicious traffic, the application can be made unavailable to legitimate users. Generally, you would protect yourself from a denial-of-service attack using a properly configured firewall. Sophisticated attacks involve sending malicious traffic from many different (or spoofed) IP addresses, making them much harder to defeat.