Boot sector viruses - Computer operating systems use part of the hard disk when starting up after the computer is turned on. This area of the hard disk is called the boot sector. When a virus infects the boot sector it can load itself before the operating system. When floppy disks (which also have boot sectors) were common they could be used to boot as well and were a common vector for infection.
This type of virus affects the boot sector of a floppy or hard disk. This is a crucial part of a disk, in which information on the disk itself is stored together with a program that makes it possible to boot (start) the computer from the disk.
Memory resident viruses - These viruses stay in memory, like the name suggests. Others just move from file to file and program and program.
This type of virus is a permanent which dwells in the RAM memory. From there it can overcome and interrupt all of the operations executed by the system: corrupting files and programs that are opened, closed, copied, renamed etc.
Polymorphic viruses - These change their "shape" to make them harder for anti-virus software to detect.
Encrypt or encode themselves in a different way (using different algorithms and encryption keys) every time they infect a system.
This makes it impossible for anti-viruses to find them using string or signature searches (because they are different in each encryption) and also enables them to create a large number of copies of themselves.
Macro viruses - Many business software programs and suites have macro language and scripting features that can be converted to use for viruses.
Infect files that are created using certain applications or programs that contain macros. These mini-programs make it possible to automate series of operations so that they are performed as a single action, thereby saving the user from having to carry them out one by one.
Overwrite Viruses
Virus of this kind is characterized by the fact that it deletes the information contained in the files that it infects, rendering them partially or totally useless once they have been infected.
Direct Action Viruses
The main purpose of this virus is to replicate and take action when it is executed. When a specific condition is met, the virus will go into action and infect files in the directory or folder that it is in and in directories that are specified in the AUTOEXEC.BAT file PATH. This batch file is always located in the root directory of the hard disk and carries out certain operations when the computer is booted.
CSS viruses - Browsers that have scripting functionality enabled can be exploited by cross-site scripting. Web sites with advertisements that come from another source (as most do) can be especially vulnerable.
Companion Viruses
Companion viruses can be considered file infector viruses like resident or direct action types. They are known as companion viruses because once they get into the system they "accompany" the other files that already exist. In other words, in order to carry out their infection routines, companion viruses can wait in memory until a program is run (resident viruses) or act immediately by making copies of themselves (direct action viruses).
FAT Virus
The file allocation table or FAT is the part of a disk used to connect information and is a vital part of the normal functioning of the computer.
This type of virus attack can be especially dangerous, by preventing access to certain sections of the disk where important files are stored. Damage caused can result in information losses from individual files or even entire directories.
Cavity viruses - These viruses hide within executable files using the "free space" within the files so that the total file size does not change and arouse suspicion.
File Infectors
This type of virus infects programs or executable files (files with an .EXE or .COM extension). When one of these programs is run, directly or indirectly, the virus is activated, producing the damaging effects it is programmed to carry out. The majority of existing viruses belong to this category, and can be classified depending on the actions that they carry out.
Directory Virus
Directory viruses change the paths that indicate the location of a file. By executing a program (file with the extension .EXE or .COM) which has been infected by a virus, you are unknowingly running the virus program, while the original file and program have been previously moved by the virus.
Once infected it becomes impossible to locate the original files.
Trojans - A trojan appears to be one type of file or program but is actually a virus.
Another unsavory breed of malicious code are Trojans or Trojan horses, which unlike viruses do not reproduce by infecting other files, nor do they self-replicate like worms.
Worms - Worms do not require user actions to spread, they move from system to system over networks on their own. For more information see my articles on computer worms, specifically the Conficker worm, and how to protect your computers and networks from them.
A worm is a program very similar to a virus; it has the ability to self-replicate, and can lead to negative effects on your system and most importantly they are detected and eliminated by antiviruses.
Logic Bombs
They are not considered viruses because they do not replicate. They are not even programs in their own right but rather camouflaged segments of other programs.
Their objective is to destroy data on the computer once certain conditions have been met. Logic bombs go undetected until launched, and the results can be destructive.
Metamorphic viruses - these rewrite themselves each time they infect a new file.
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