Ransomware implies certain computer virus program which hijacks the entire computer system, or the information it contains, and takes it hostage against its user by requiring user to pay some ransom price for its restoration. Ransomware enters system by means of, for example, some vulnerability in a network service or an e-mail attachment. It may either disable some type of essential system service or lock the desktop at system startup. It may also encrypt some of the user's personal files. In all such cases, the malware may prompt the user to enter a code obtainable only after issuing payment to the attacker or sending an SMS message and accruing a charge. It may also require from the user to purchase a decryption or removal tool.
Rogue security software (also referred to as rogueware) implies certain PC malware application aimed to deceive or to scare users into purchasing the bogus or fake program for alleged removal of viruses and malwares which don't really exist. In some cases the rogue tool also installs other malware. It should be noted that rogue security programs, over the latest period of time (2008–2011), have become a growing and serious security threat for desktop and laptop computers.
Trojan horse, more often referred to as Trojan, means certain software that seems to perform a desirable function for the user before running or installation, but, in addition to the expected function, is engaged in stealing of private information or making harm to the system. It should be emphasized that this term is derived from the Trojan Horse story of Greek mythology.
Zeus is the implication of certain Trojan horse which is aimed to steal banking information by means of keystroke logging and Form Grabbing. It should be mentioned that Zeus is typically spread by means of drive-by downloads and phishing schemes.