Actually, a (D)DOS is an attack on a computer with the goal of making it unable to do it's intended operations. It doesn't even need to be on a network but most of the time it is, and most of the time, physical attacks aren't called DOS.
A DDOS is just a DOS using multiple machines on the internet at the same time. No need to send buggy packets or something : if a server is built to handle at most 100 page views per second, you take 1000 zombies on the net who will try to retrieve 1000 of times per second the web page and the server will meltdown in the operation. For those who know what it is, the slashdot effect would probably be considered as a DDOS if the melting down the webserver was wanted
