A virus (computer virus) is a piece of software that is designed to intentionally cause damage to a computer. It has the ability to copy itself and infect different files and computers. Depending on the mode of operation, we can define several types of viruses:

• Spywares are the programs that collects small pieces of information about users without their knowledge. They are intended to monitor/Log the activity performed on a computer. These are used to spy on some one either for legal or illegal perpose.
• Keyloggers (use Keystroke logging) are examples of spywares. This type of software track (or log) the keys struck on a keyboard, typically in a covert manner so that the person using the keyboard is unaware that their actions are being monitored.
• Malwares or malicious software are programs with malicious intention. This term defines all the unwanted and malicius software installed in a computer (viruses if you want). This software can damage your computer, spy on you or perform any other malicious task.
• Adwares are unwanted programs that automatically plays, displays, or downloads advertisements to a computer (generally in POP-UP’s form). They are not intended to harm your computer. The object of the Adware is to generate revenue for its author.
• Trojan Horse is a program which is used to gain access to a computer. It’s main function is to open doors for unwanted external access. Trojans are also known as Backdoors. They may allow a hacker to remote access to a target computer system.
• Worm are known in the context of computer networks. They are programs that self- replicate and uses a computer network to send copies of itself to other computers and it may do so without any user intervention. They often slow down the network.

Photo: [Nick]