DoS or DDoS can refer to a "Denial of Service" or a "Distributed Denial-Of-Service" attack.
A DoS Artist is a person that will attempt a DoS attack on a server and tell their friends that they actually hacked the server. This type of person makes their entire "hacker life" all about staying holed up in their rooms to try this attack on other people or servers, usually so they can brag about it to their friends some more. These types of people are also often associated with "Script Kiddies"
'John' isn't a hacker, he's a total DoS artist... That's all he does.