types of omputer virsus

Viruses can be catagorized in more than one way.  For example, they can be catagorized by their primary function and propagation method as follows:
Trojan horse--enters a system disquised as something else
Worm--propagates on its own by a variety of means including hijacking email accounts, user ids, file transfer programs, etc.
Bomb--doesn't propagate itself at all, is placed by a human or another program and activated by a trigger such as time or event. Usually does something unpleasant when it goes off.
Port Scanner--hides on a system and scans the surrounding environment for IP addresses and open ports that it then makes available to other malicious code or individuals.

The way viruses are usually catagorized however, is by what they do as follows:
Boot Virus--infects the boot sector of disk storage  (Form, Disk Killer, Michelangelo)
Program Virus--infects executable programs (Sunday, Cascade )
Multipartite Virus--combination of the first two (Invader, Flip, Tequila)
Stealth Virus--able to avoid detection by a variety of means such as removing itself from the system registry, masqarading as a system file, etc. (Frodo, Joshi, Whale)
Parasitic Virus--embeds itself into another file or program such that the orginal file is still viable (Jerusalem)
Polymorphic Virus--changes its code structure to avoid detection and removal, mutates (Stimulate, Cascade, Phoenix, Evil)
Macro Virus--exploits the macro language of a program like MSWord or MSExcel for malicious purpose (DMV, Nuclear, Word Concept)
Hope this is helpful