DDoS
A DDoS attack is an attack where a server is flooded beyond its processing power (bandwidth and maximum simultaneous connections, typically) to take it down. Connections are spammed to a server, particularly from multiple sources (a DDoS from a single source is called a DoS), in an attempt to take it down. This is common across the internet, and DDoS attacks are often launched against anything from webservers to game servers.
In Jedi Academy, DDoS attacks got so bad that Caelum wrote an article on the subject on JKHub, DDOS attacks and their effects on the community, which elaborates more on the effect on Jedi Academy servers and players.