They are just mini IRC clients (usually only a few KB's in size) that propagate via a website set up to automatically infect a user when the site is visited.
One recently experienced by my network was Lipreffs.Worm which I understand to be primarily set up to hammer the security related website http://grc.com/
yet that trojan also has a client which 'follows' your mIRC (I don't know how but it does) to the server you connect to effectively being a second connection. It doesn't join any rooms but DOES advertise via /msg to anyone joining any of the rooms you are present in. The setup is quite clever but very destructive. There are at least 7 Geocities websites that I know of that are infected with this trojan and possible dozens more than I don't know about because whoever has rebirthed this trojan means business. They want to nail GRC again.
The Dalnet problem is a bit similar but uses a trojan called ROL.vbs or a similar infection. It simply infects your mIRC directory with a bad text file which can (depending on the version) either just advertise itself, which invariably lures more chatters to the be-trojaned website, or can contain flood scripts, a botnet script where via a socket you are joined to some distant IRC server without any knowledge of doing-so and you join a channel where the master is. When enough infected people join his channel he can control the potentially 1000's of bots and can command them to join any server on any network and flood any channel he likes or just simply flood a server with useless /server and /quit commands until the server says "I've had enough" and crashes.
My network was attacked a while back by someone wanting to set up a botnet channel. Shame this network doesn't allow users to make their own room, they have to be applied for, which is probably why we enjoy a comparitivily increased level of peace and quiet. The senior admin told me there was around 2500 unique connections. It doesn't sound alot and it's about half of what Dalnet have experienced (quoting from their website) but 2500 connections every few seconds does place a big load on a server.
Is it easy to stop them connecting? Yes In time it is.
Is it easy to stop them trying to connect? It can be but not always. You need nous, teamwork and the will to do it.
Networks will of course be affected differently. It depends on the money spent on equipment the size of the connection, the software used and the features the software contains.
Who's next to cop a hammering from the flooders? Well both here and on several IRC related websites it has been noted and predicted that all the big networks are potential targets because the bigger the trophy the better the flooders will feel. That's understandable. Can another networks pending demise be prevented? Only the man upstairs knows the answer to that one. It will depend on how well the rest of the IRC community is prepared to try and stop it happening. For this to happen they MUST ACT NOW and plan for an attack even if it never happens.
What networks have been attacked in the last year? Heaps. Dalnet, Efnet, Austnet, Webnet, Telstra, and scores of others have been nailed and I mean nailed hard. Dalnet was the biggest of those though and is probably the main reason for the continued attacks. There's a report on IRCnews about infighting among Dalnet opers. If this is happening they will have to pull themselves together and work as a united front or the network will die.
Can the culprits be caught? Yes but the only chance of that happening is if they get complacent and 'slip-up' somewhere. They are obviously disguising their bona-fides when making the infectious websites. If they achieve this then they get off scot-free, simple. To be truthful, while it is a perfectly reasonable thing to try and track down the villians I personally think it is a waste of time and resources. Put the resources into making the network infrastructure more resilient to attacks therefore giving the users a more stable network is a better option. To be frank, I don't think either Dalnet owners or the chatters there care anymore about it being part of the Big 4. Just existing should be the main priority.