Malware is software that is designed to infiltrate or damage a computer system without the owner's informed consent. It is a combination of the words malicious and software. The expression is a general term used by computer professionals to mean a variety of forms of hostile, intrusive, or annoying software or program code. Viruses, worms, Trojans, and bots are all part of a class of software called malware.
Types of malware:
Viruses A computer virus propagates by inserting a copy of itself into and becoming part of another program. It spreads from one computer to another, leaving infections as it travels. Viruses can range in severity from causing mildly annoying effects to damaging data or software and causing denial-of-service (DoS) conditions.
Almost all viruses are attached to an executable file, which means the virus may exist on a system but will not be active or able to spread until a user runs or opens the malicious host file or program. When the host code is executed, the virus code is executed as well.
Normally, the host program keeps functioning after it is infected by the virus. However, some viruses overwrite other programs with copies of themselves, which destroys the host program altogether. Viruses spread when the software or document they are attached to is transferred from one computer to another using the network, removable media, file sharing, or infected email attachments.
Other common virus types:
Macro virus
A virus that exploits vulnerabilities in programs like Microsoft Word, Excel, and Outlook causing certain actions to perform automatically, usually triggered by selecting some other action in the program or when the program starts.
Retrovirus
A virus that is designed to avoid discovery by actively attacking the very anti-virus programs attempting to detect it.
Worms
Computer worms are similar to viruses in that they replicate functional copies of themselves and can cause the same type of damage. In contrast to viruses, which require the spreading of an infected host file, worms are standalone software and do not require a host program or human help to propagate. To spread, worms either exploit a vulnerability on the target system or use some kind of social engineering to trick users into executing them.
A worm enters a computer through a vulnerability in the system and takes advantage of file-transport or information-transport features on the system, allowing it to travel unaided.
Trojans
A Trojan is a harmful piece of software that looks legitimate. Users are typically tricked into loading and executing it on their systems. After it is activated, it can achieve any number of attacks on the host, from irritating the user (popping up windows or changing desktops) to damaging the host (deleting files, stealing data, or activating and spreading other malware, such as viruses). Trojans are also known to create back doors to give malicious users access to the system. Unlike viruses and worms, Trojans do not reproduce by infecting other files nor do they self-replicate. Trojans are spread through user interaction such as opening an email attachment or downloading and running a file from the internet.