Varieties of computer viruses.
Worm. This computer virus litter your computer.This virus is practically not dangerous. Your computer will fall behind because the virus copies itself, taking up free space on the HHD or SSD. The worm can not become part of any harmless program.
trojan horse (trojan). As you can understand from the name - the virus is masked in harmless programs. The virus does not harm the computer, until the moment when the user does not launch the program in which the virus is hiding. This type of virus is used to steal, delete or modify data.
Programs are spies. This type of virus basically hunts for your passwords. But it can also track your behavior. That is, to collect all the necessary information about what sites you visit, and so on.
The viruses are zombies. This type of virus, takes over your computer, allowing you to use it for the purposes of the attacker. By the way, you can not even guess that there may be such a virus in your computer.
Virus blockers. This virus blocks the user's access to the operating system. Surely many faced such viruses. If after loading the operating system there is a window with threats and requirements to pay the extortioner for unlocking, you caught this type of virus. In principle, after paying you no one will return access to the operating system.