A Trojan horse meets the definition of virus that most people use, in the sense that it attempts to infiltrate a computer without the user’s knowledge or consent. A Trojan horse, similar to its Greek mythological counterpart, often presents itself as one form, while it is actually another.

Trojan horses typically do one of two things: they either destroy or modify data the moment they launch, such as erase a hard drive, or they attempt to ferret out and steal passwords, credit card numbers, and other confidential information.

Trojan horses can be a bigger problem than other types of viruses as they are designed to be destructive or disruptive, as opposed to viruses and worms where the coder may not intend to do any harm at all. Essentially this distinction does not matter in the real world. You can lump viruses, Trojans, and worms together as "things I don't want on my computer or my network."