Computer Virus
A computer virus is a program, file, ect. that can infect your computer, and can delete essential files or programs. Some viruses can be a hoax or joke, others can be vital to your computers system. The differences between the other malicious things on the Internet (for example trojan horses), is that viruses can clone themselves to make them harder to get rid of. Some viruses can bundle multiple malicious files with them.
Types
Trojan Horse
The goal of a Trojan Horse virus is to install itself secretly, to allow a backdoor to your computer. Thus, it could now do a number of things, botnets, malware, Adware, keyloggers, etc. Computers are usually infected by downloading them bundled with some other software.
Keyloggers
A program that records all of your keystrokes, and sometimes even mouse movements, and images of your screen. This is obviously bad, since it records your financial information, passwords, and the like.
Adware
Main Article:Adware
A virus that plays, or shows unwanted ads. An example would be pop-ups/pop-unders. It typically slows down your computer. Some Adware display ads that have links to malware.
Spyware
A special type of Malware, which primary purpose is to spy on your computer. A form of this is a keylogger (see above).