Denial of service (DoS)
The attempt to 'bring down'(disable it to deliver pages) a website. Normally this is done by feeding more traffic into the webserver as it may be able to handle. For this you need either a huge bandwidth to attack from (using regular page requests) or you use 'incomplete' requests to produce more requests using a given bandwidth.
Firewalls can cope with this incomplete packet sequences if they do a 'stateful inspection' (the only let complete, valid sequences of packets pass).