"Trojan horse" programs are similar to viruses in their effect on your system, but they can't reproduce themselves.
They're usually a program disguised as something you might want to download onto your computer—for instance, a rogue, modified version of PKWare's PKZIP utility.
These program codes have gone beyond mere annoyances with the worst kinds disabling your PC, but they have become portals for remotely perpetuating more sinister activity that can clandestinely hack into sites, mount denial of services or steal confidential and personal data for fraudulent financial gain at your expense. Among home PC users, you may think having to reinstall your OS after a virus or malware has brought it down is not really expensive as you lose just a day or two to reinstall your programs and rebuilding files, consider that in a business, you could actually lose millions.
Just ask Choice Point when it took a $6 million charge in 2005 after cyber criminals hacked into their systems and stole sensitive data from thousands of customers.
What it does then depends on the malevolence of its creator.