Malware: Defined
Malware (short for "malicious software") is any program or set of programs designed with harmful or malicious intent, such as spyware, worm-based spam mailers, viruses that subvert the host computer or degrade and prevent the intended use of a network, adware that automatically re-installs itself when removed, and many other cases.Grayware (or greyware) is a general term sometimes used as a classification for applications that behave in a manner that is annoying or undesirable, and yet less threatening than malware. Grayware is not designed with malicious intent, but can negatively affect the performance of your computer and introduce security risks. Often grayware performs a variety of undesired actions such as irritating users with pop-up windows, tracking user habits and unneccesarily exposing computer vulnerabilities to attack. (This definition of Grayware is akin to PUPs -- Potentially Unwanted Programs.)
Viruses and worms are known for the manner in which they spread, rather than any other particular behavior. The term computer virus is used for a program which has infected some executable software and which causes that software to spread the virus to other executable software. Viruses may also contain a payload which performs other undesirable actions. A worm, on the other hand, is a program which actively transmits itself over a network to infect other computers. It too may carry a payload.
These definitions lead to the observation that a virus requires user intervention to spread, whereas a worm spreads automatically. Using this distinction, infections transmitted by email or Microsoft Word documents, which rely on the recipient opening a file or email to infect the system, would be classified as viruses rather than worms. However, this strict interpretation has become blurred by common misuse. Infectious code may also be written in the macro language of Microsoft Word and similar programs. These macro viruses infect documents and templates rather than applications, but rely on the fact that macros in a Word document are a form of executable code.
Adware and Spyware programs do not spread like viruses; they are often installed by exploiting security holes or comes packaged with user-installed software, such as peer-to-peer applications, or masquerading as some other type of application or file. Typically, a single small "trojan downloader" will infect your computer through one of these means, and then download the actual spyware and adware.