from what I know (I am not an expert, still studying
) the typical one is a trojan little program that can use your connection to flood a specific target (not long ago someone tried to do this to the site of microsoft that distributes the patches for microsoft products)
Once you have this trojan in say 3000 computers, odds are that say 1000 might have high speed connections that in the States for example can have an average of 256 kbps (its the upload part not the download speed that counts here)
so imagine 256 kbps times 1000 plus 33 kbps times 2000 and you have killed a normal site
also depending on how skilled the attacker is, he can "spoof" that is fake the ip that is sending the info , so you can't even trace the thousands of machines that are targetting you
There are many ways to mitigate these attacks and its not the first time that they happen
if you are interested take a look at this link
http://grc.com/dos/drdos.htm (there is a 23 page paper on the subject), the explanation there is very complete and easy to understand so everybody knows what happens when these idiots start to attack