I have a friend of mine who told me that all computer viruses were the same, which is of course not true, when I told him that different there are several types of computer viruses and that they had their own classification, he didn't believe me, so in honor of him I am making this small article about the different types of virus classifications there are, at least the ones I'm aware of.
Boot Virus: This virus pretty much affects the boot sectors of the hard disk, specifically the area where the information that the computer uses to boot up from is located. To avoid this type of virus you would simply make sure that there were no floppy disks in the computer when you booted it up, or if there were, that they were write protected.
Resident Virus: These viruses are present in the computers RAM (Random Access Memory) and because of that they are able to interrupt, intercepts and overwrite operations that are executed by the system, infecting files that are opened, copied, closed, and renamed.
Macro Virus: These were a big problem back in the late 90's/early 2000's due to the large use of MS Word and other Office type programs that allowed macros to be embedded in their documents. These allowed multiple actions to be executed without the user ever catching on, while most where annoyed, some actually had destructive capabilities.
Polymorphic Virus: Back in the days when Norton AntiVirus utilized inoculation files in directories to stave virus activities polymorphic viruses were almost always the exception. These types of viruses change their size, file type, and/or properties with each infection they cause. They are able to re-encrypt or re-encode themselves in order to avoid heuristic virus detection and make it difficult for anti virus scanners to keep tabs on.
Trojan Horse Virus: These viruses disguise themselves as legitimate programs or even harmless prank programs that, once executed, infect the computer and either cause destructive damage or allowed a hacker bookdoor access to the machine.
Logic Bombs: These types of "viruses" are tid bit complex in that they are not really viruses,they are not even considered programs really. They are pretty much segments of code that eventually execute a command once a serious of prerequisites have been met, and 9 times out of 10, the results are devastating.
Well, there you have it, a nice little article on the different classifications of viruses there are that I can recall.