Answer:
"Denial of service attacks" is when someone is trying to hack into your system using some sort of software that is trying to guess the username/password...
For example, in Windows XP find Event Viewer under Computer Management in the Control Panel. Then look for System Viewer; it will show who logged in, who logged out, and if someone is constantly trying to log in. If a Network Administrator finds someone who tried to log in, say, 100 times, then he can block that IP address from logging in at all.