DDOs = distributed denial of service 
a denial of service attack is an attack at servers which can't be stopped. When you try to gain acces to the server the server wil return a denial of service. People exploit this. They send as many service request as possible. The server then needs to send a denial of service for all requests. Due to this a lot of bandwith is used and no bandwith is left over for normal traffic (websites, mail, irc, ...) 
the distributed means the attack is comming from multiple machines this to increase bandwith on the attack so that absolutly no one can access the server anymore. 
In the beginning of this attack we saw that some people (including me) could still rejoin our IRC server. But once another machine helpd attacking it is impossible to gain access to the server. 
This attack is not directed @ mta but at our host or it's ISP.