MS05-027 is not categorized as virus, worm, Trojan or backdoor. It is a critical vulnerability in Server Message Block (SMB) in Windows 2003/XP/2000, which allows to remotely execute arbitrary code in the vulnerable computer.
If exploited successfully, MS05-027 allows hackers to gain remote control of the affected computer with the same privileges as the logged on user. If this user had administrator rights, the hacker could take complete control of the system: create, modify or delete files, install programs, create new user accounts, etc.
There are several known attack vectors in order to exploit MS05-027: create special network packets and send them to a vulnerable computer, or use a program that passes parameters to the SMB component.
Attacks can be launched across the Internet, but a firewall could be used in order to prevent those attacks.
If you have a Windows 2003/XP/2000 computer, it is recommendable to download and apply the security patch for this vulnerability. Click here to access the web page for downloading the patch.