Type of Viruses

There are various types of viruses :

Boot viruses place (some of) their code in the disk sector whose code the machine will automatically execute when booting. Thus, when an infected machine boots, the virus loads and runs. After boot viruses are finished loading, they usually load the original boot code, which they have previously moved to another location, or take other measures to ensure the machine appears to boot normally.
File viruses attach to 'program files' (files containing executable or interpretable code) in such a way that when you run the infected program, the virus code executes. Usually the virus code is added in such a way that it executes first, although this is not strictly necessary. After the virus code has finished loading and executing, it will normally load and execute the original program it has infected, or call the function it intercepted, so as to not arouse the user's suspicion.
Macro viruses are really just a type of file virus, but a particularly 'successful' type. They copy their macros to templates and/or other application document files. Although 'auto macros' were almost exclusively used by early macro viruses (often to ensure the virus' code is the first to execute when infected templates or documents were opened), several other mechanisms are also available - in fact, some of these, such as taking over standard internal functions of the host application (say the 'File Save' command) and installing default event handlers are probably more commonly used these days.
Script viruses also became quite successful around the beginning of this century. This was mainly due to the increase in machines running Windows Scripting Host, which was first installed by default in Windows 98 and 2000 and with Internet Explorer 5.0 and later versions. Representing new types of 'program file', but with icons more like that of 'safe' text files, standalone Visual Basic Script (VBS) and JavaScript (JS) programs became a popular target of the writers of mass mailing viruses.
Companion viruses take advantage of features of the operating system to be executed, rather than directly infecting programs or boot sectors. Under DOS and Windows, when you execute the command 'ABC', the rule is that ABC.COM executes before ABC.EXE (in the rare cases where both files exist). Thus, a companion virus could place its code in a COM file with its first name matching that of an existing EXE file. When the user next executed the 'ABC' command, the virus' ABC.COM program would be run (usually the virus would launch ABC.EXE once its code was finished so as not to arouse suspicion). This is known as the 'execution preference companion' method, but several other forms of companion infection are also possible.