A computer virus is a item that can infect your computer and delete essential files or programs. Some viruses can be a hoax or joke, others can be vital to your computer's system. The differences between the other malicious things on the Internet (for example a Trojan Horse), is that viruses can clone themselves to make them harder to get rid of. Some viruses can bundle multiple malicious files with them.
Main Article: Trojan Horse
A Virus that installs secretly to allow a backdoor to your computer. Thus, it could now install Botnets, Malware, Adware, Keyloggers, etc. Computers are usually infected by downloading it as a bundle.
Main Article: Adware
Main Article: Spyware
A term used to describe software that performs certain behaviors without appropriately obtaining your consent first, such as advertising, PII harvesting and changing the configuration of your computer.
Main article: Keylogger
Programs that record all of your keystrokes, and sometimes mouse movements and images of your screen. This is obviously bad, since they record your financial information, passwords, and the like.
Avoiding getting infected
Have a regular scan of your computer with an anti-badware product like these products.
You might also want to see this topic on what the WOT community has to say.
The HOSTS file
You can also block harmful websites too, using the HOSTS file, located at C:\WINDOWS\system32\drivers\etc
Example: typing something like "127.0.0.1" or "website.com" would result in "website.com" always redirecting to "127.0.0.1" (localhost).
Be sure to not to save your HOSTS file using "Save As", because it won't work if the extension is .txt