Denial of service can occur when someone overloads a computer system or network with bogus requests. One writer received 20,000 e-mail messages in a single day. Imagine trying to find your own e-mail messages if someone sent you twenty thousand messages you did not want. It would take hours just to read through the subject lines to find the e-mail you wanted. That would be a denial of service.
Denial of service is a breach of availability.
Finally, the last principle of information security is utility. Utility means usefulness. For example, if you went to the local store and all of the prices were in Norwegian Kroner but nobody knew how many Kroner there were in the dollar, that would not be very useful to you. Now that you have some idea of what information security people worry about, it's pretty easy to understand why breaking into somebody's computer system is really bad. Of course the obvious problems concern confidentiality, integrity and authenticity.