A trojan horse or Trojan Horse Virus is not a virus rather it is malware.
Malware is a combination of the two words malicious and software = malware, to accomplish its goals, it must be able to run without being detected, shut down, or deleted.
Malware is software that the user either deliberately or unintentionally installs on their computer. Malware objectives started as benign software that installed unwanted advertising but quickly developed into software that’s purpose was to steal sensitive personal, financial, or business information for the benefit of others (2006, McAfee White Paper). []
The software is often hidden in legitimate software provided through unscrupulous servers. It appears to be useful or legitimate but provides a ‘back door’ that allows a third party to access previously protected computer without permission and extract, data, private or protected information. Malware can also can corrupt existing data, act as a ‘key logger’ and extract sensitive information as it is being entered, or act in consort with other computers to commit cyber crimes (IE overloading a servers). According to Mark E. Russinovich a Technical Fellow in the Platform and Services Division at Microsoft, for a malicious program to accomplish its goals, it must be able to run without being detected, shut down, or deleted [] . Trojans require Rootkits to avoid detection and Backdoors are often used to bypass authentication and provide access for the ‘unauthorized user’. By definition a Trojan horse is any program that invites the user to run it, concealing harmful or malicious code. The code may take effect immediately and can lead to many undesirable effects, such as deleting the user's files or installing additional harmful software. Rootkits assist this subterfuge , by modifying the host's operating system so that the malware is hidden from the user. Rootkits obscure the process so when running processes are checked the malware is invisible. Lastly backdoors are often installed by the Trojan Horse to allow the previously unauthorized user a quick way to access and retrieve the data harvested by the Trojan Horse. [] Some malware contain routines to defend against removal, to not just obscure or hide themselves, but to actively prevent attempts to remove the software from the system.
Modified (11-7-2012 R Lettvin)