Long, drawn out explanation:
https://security.stackexchange.com/a/22816Short: you're trying to overload a server by any method that a server attempts to accept. This isn't the best explanation, but it'll do: It's like throwing more then 2 balls to someone and expecting that person to catch every ball you throw without putting the other balls down. Nostalrius is like having 5k - 10k people trying to catch all the balls. When someone attempts a DDoS attack, balls are coming from everywhere and by the truckload. From Japan, China, Ukraine, France, US, Canada, Mexico, Greenland, and whatever other countries that have "slaves" (infected networks/computers from malware) to attack a target (Nostalrius).
The reason for doing this? Someone is probably angry because they had their account banned. Is there a permanent defense against this? No, it's a flaw in how packets are sent and received. You can mitigate it, but you can't stop it unless you assassinate the person doing this.