"DDoS" stands for "Distributed Denial of Service.

It's a method of sending so much data to one computer / IP Address, usually, but not always, a simple "ping", that it essentially gets flooded, can't handle it, and either gets completely knocked offline, or becomes so slow that for all practical purposes that computer, or even website, becomes unusable.

Ever hear of a DOS program? I know someone who is having one used against her system at home and she doesn't know how to stop it.
She's saying some kid got a hold of her home ip address and is using a DOS (denial of service) program to bombard her router or modem or whatever so she cant get service. Everything is bogged down.