There are many types of computer viruses or malware (malicious software): -

Virus - A small software program that infects your computer, attaches itself to an existing program and corrupts your system.

Trojan horse - Presents itself as a free and useful software program, but once it is on your system it can give criminals access to your files and information.

Spyware - Silently installs itself on your computer and sends information about you and your personal data back to the criminals.

Adware - Designed to display advertisements on your computer, but can also be used as Spyware.

Keylogger - Monitors every keystroke that you make; used by some companies to monitor their employees, but can also be used as Spyware.

Hijacker - Interferes with your Internet browsing software (Internet Explorer) and redirects you to websites displaying advertisements rather than your normal home page.

Fake security software - Disguises itself as a anti-virus or virus scanning software program that can be downloaded for free, but once installed it will report various non-existent security issues on your system.

Once a virus is on your system it is not always possible to remove it using your own anti-virus software and the effect that the virus has on your computer depends on the type of virus (or viruses) that have infected it.