It is so much kinds. I teach you representative types.
Virus of infected of file
It is diving in extension, for example, [exe] [com]. It works starting a program and breaking the data.
Virus of infected of macro
Word and Excel have macro program. Macro learns fixed process and repeat timing when you want to do. Virus of infected of macro hides macro program. It works starting a macro program.It is burden why it works any OS.
Virus of infcted of boot sector
Boot sector is organized hard disk srive to import the proram starting OS. Virus of infected of boot sector does force control.
A worm is the only increacing program. It moves by way of mail.
It fakes useful program myself. It looks useful program when it downlord. It works using the program. It is named Trojan horse Troia War.
It works special condition. Almost it break program together with OS. It means that you can't use the computer. Chernobyl is one of famous logic bom.
It steels FTP password. It is dificult to deal with attacking WEB site.
It steels your name, password and mail address instead of you realizing that. It is dangerous not to influence your computer.
It deprives your authority and operate without permission. When you derives your authority, it is used crime.