Netbus, also called Netbus Trojan, is a Win32-based Trojan program. Authored by Carl-Frederik Neikter, it first appeared in March 1998 and is one of the most famous Trojans created. Netbus Trojan needs to be executed by the user for it to be installed. Once executed by the user it will install itself in such a way that it will be active all the time by adding an entry into the Windows registry. While you are connected to the Internet, if this program is running on your computer, anyone who has got the Netbus client can sneak in to your computer without your permission or knowledge.
Also see The Difference Between a Virus, Worm and Trojan Horse in the Did You Know? section of Webopedia.