DDOs stands for Distributed Denial Of Service, it basically is when the 'hacker' infects peoples computers with a botnet virus (some are fairly common and you can get source codes if you google) others are private though and can cost thousands for the code etc. Anyway getting to the point they are usually written in C# or C++ some are vb etc and they essentially are used to make the victims into zombies. Once the 'hacker' infects the victim usually via p2p etc or by binding it to an exe file the victim joins the 'hackers' irc and that is where the bots reside. If i was to log into my theoretical botnet now my bots would be all listed in the irc chanell id then command them according to how the bot is coded e.g.
!login passwordhere
once logged in i would inform the bots what to do e.g.
SYN
www.buddforum.de 800(thats the port you want it to atack)
the command would then be sent to all of the zombies and they would on a loop continiously syn atack / packet flood the website making it go slow.
Botnets also let you get info such as password dumps, keylog files, screenshots of the victim, credit card info and lots of other stuff. Most botnet code is detected though so they are usually encrypted. Some botnets connect via html my friend once coded one in vb.net that worked via html (didnt spread it though alls well).
Most botnets have a few hundred k zombies usually as you can command it to spread depending on how the bots coded but if its a half decent one it probably has usb/p2p/msn spreading etc. They can be pretty dangerous and spread like wildfire and thats just the ones script kiddies can download and configure... ones coded by people and kept private may never even be discovered by antivirus databases etc so they could remain hidden and operating without you knowing.
I hope I helped you all have a better understanding of how a DDOs atack works and what it is.