Trojan Horse
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Trojan Horse

A computer program that has a useful function, but which also contains additional (hidden) functions. The hidden functions may secretly exploit the legitimate authorizations of the invoking process to the detriment of the system's security; for example, by making a blind copy (BCC) of a sensitive file for the creator of the Trojan Horse. Trojan Horses are often designed to deliver a tool (perhaps a RAT) that may then be used to subvert a system's security mechanisms; see, for example, Back Orifice.

There is a fine line between a Trojan Horse and Adware.

Trojan Horse File
Legend says that after a prolonged war between the Greeks and the Trojans, the Greeks finally surrendered, left behind a gift of truce, abandoned Troy and set off home. Their gift was a large wooden horse, which the Trojans accepted eagerly (despite the advice of their chief priest, who suspected a deception). The horse was larger than the city gates, which were duly demolished. The statue was wheeled in and the party commenced.

In fact, the Greeks had merely sailed around the closest headland to await nightfall. Also, a secret compartment inside the horse contained a select band of Greek fighters, who duly broke out under cover of darkness to initiate the slaughter. Their countrymen sailed back, piled in through the breached defences and completed the rout. Troy was lost.

And that is why it is generally a bad idea to run a program, look at a document or use a spreadsheet that someone has sent you, especially if you do not know that person, or if you were not expecting them to send it to you. If it comes from an untrusted person, then it has untrusted content, whatever words of comfort they may offer you about it. Listen to the chief priest of the Trojans. Discard it.

Even if it seems to come from someone you trust, do not use it if you were not expecting it. There are viruses (see Library File) which send email attachments as if they were you, thus lulling recipients into a false sense of security. Don't feel bad about refusing files with programmatic content from your friends and colleagues. You can usually get the programs you need from your administrator instead of your friends, and you can ask to receive documents in RTF (see RTF) instead of accepting DOCs.

from Sophos' V-Files