What kinds of computer viruses are there?

Backdoor: A virus which aims to compromise the machine for some type of intrusion attack or to capture and send out proprietary information, such as user names and passwords, contained on it. See also 'Trojan Horse'. Boot Virus: A type of virus that infects PC boot sectors or the Master Boot Record. This type of virus is usually contained on a floppy and can only infect the PC when the machine is booted with the floppy inserted. These are some the earliest types of viruses since older machines used floppies to a great extent. Bug: A software error. The consequence of a bug often resembles a virus; bugs can cause the machine to crash, lock-up, report unusual error messages, and more. Unlike a virus, a bug represents a mistake, not a deliberate attempt to cause computer problems. Hoax: A hoax is just that. Hoaxes tend to spread through chain emails. A hoax will claim everything from the existence of a new and destructive virus, to the desperate plea for help for some poor fellow. ...  more