DoS (Denial of Service) is simply rendering a service incapable of responding to requests in a timely manner. This is a controversial subject, since some people think that DoS is not a hack, and/or is rather juvenile and petty. We prefer to think of them as just one more kind of tool in the toolbox, and as such, will continue to include material on them in the Hack FAQ. Ask yourself which is more alarming - the number of kids trying DoS attacks, or the number of DoS attacks that succeed?
Regardless of your feelings, DoS has been steadily gaining in popularity, whether with hackers mad at other hackers, sysadmins mad at spammers, or whatever - virtually everyone we've run into that is aware of the potential of DoS at least has software to do it, admins included.