What are Adware, Spyware & Viruses?

Adware:

a form of spyware that collects information about the computer user's online behavior in order to display targeted advertisements in the Web browser. Software to remove adware can be found on this page.

Spyware:

Software that transmits information without the user's knowledge.Information is sent via the Internet to a server somewhere, normally as a hidden side effect of using a program. Gathering this information may benefit the user indirectly, e.g. by helping to improve the software he is using. It may be collected for advertising purposes or, worst of all, to steal security information such as passwords to online accounts or credit card details.Spyware may be installed along with other software or as the result of a virus infection. Many tools are available to remove various forms of spyware and adware from a computer.Some HTTP cookies could be considered as spyware as their use is generally not made explicit to users. It is however possible to disallow them and some are actually useful, e.g. recording the fact that a user has logged in. Software to remove spyware can be found on this page.

Virus:

A program or piece of code written by a cracker that "infects" one or more other programs by embedding a copy of itself in them, so that they become Trojan horses (virus). When these programs are executed, the embedded virus is executed too, thus propagating the "infection". This normally happens invisibly to the user. A virus has an "engine" - code that enables it to propagate and optionally a "payload" - what a virus does apart from propagating. It needs a "host" - the particular hardware and software environment on which it can run and a "trigger" - the event that starts a virus running. Unlike a worm, a virus cannot infect other computers without assistance. A virus is propagated by vectors such as humans trading programs with their friends. The virus may do nothing but propagate itself and then allow the program to run normally. Usually, however, after propagating silently for a while, a virus starts doing things like writing "cute" messages on the terminal or playing strange tricks with the display (some viruses include display hacks). Viruses written by particularly antisocial crackers may do irreversible damage, like deleting files.By the 1990s, viruses had become a serious problem, especially among IBM PC and Macintosh users (the lack of security on these machines enables viruses to spread easily, even infecting the operating system). The production of anti virus software has become an industry. Many lusers tend to blame *everything* that doesn't work as they had expected on virus attacks. Accordingly, this sense of "virus" has passed into popular usage where it is often incorrectly used for a worm or Trojan horse. Software to remove a virus can be found on this page.