This article is from the the Brief Linux FAQ (BLFAQ) Maintained by Neil Zanella firstname.lastname@example.org
This is a security feature meant to protect you. Malicious users must know a valid user password, as well as the root password in order to get in to the system and do anything as root. Log in as a user, and use the su command to then become root. Better still is to use the ssh command (if installed) which encrypts your session to prevent anyone from seeing the root password. If your network is totally local (you have only a terminal connected to a serial port), you can edit /etc/securetty to add that port as one considered secure enough for root to log in.