For a malicious program to accomplish its goals, it must be able to run without being shut down, or deleted by the user or administrator of the computer system on which it is running. Concealment can also help get the malware installed in the first place. When a malicious program is disguised as something innocuous or desirable, users may be tempted to install it without knowing what it does. This is the technique of the Trojan horse or trojan.
In broad terms, a Trojan horse is any program that invites the user to run it, concealing a harmful or malicious payload. The payload may take effect immediately and can lead to many undesirable effects, such as deleting the user's files or further installing malicious or undesirable software. Trojan horses known as droppers are used to start off a worm outbreak, by injecting the worm into users' local network.
One of the most common ways that spyware is distributed is as a Trojan horse, bundled with a piece of desirable software that the user downloads from the Internet. When the user installs the software, the spyware is installed alongside. Spyware authors who attempt to act in a legal fashion may include an end-user license agreement that states the behavior of the spyware in loose terms, which the users are unlikely to read or understand.
Trojans are most commonly used for marketing. Today's advanced malware/trojans are fully capable of hijacking complete control of a browser regardless of IE or Mozilla. It has even been known to add false exceptions to a browser's security settings as well as editing a computer's registry.