MS05-011 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, MS05-011 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 MS05-011: create special network packets and send them to a vulnerable computer, an e-mail message that contains a link to a URL, 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.