The term adware is most often used to refer to any software that displays any types of advertising with or without the users consent. There are many types of programs out there one such program is Eudora mail client display advertisements as a change to shareware registration fees. These such programs are classified as adware but only in the way that is shows off advertisements. This type of adware does not mislead the user it just brings them a lot of advertisements, it is meant for more of an annoyance.
Other spyware behaviors include reporting on websites the amount of time the user has visited that site. The information is then used for advertising impressions. The importance of this spyware has thrown a doubt on other programs that track web browsing for either statistical or research reasons. Software has been packaged with free advertising and supported programs such as the P2P act as spyware. Recent test results show how a bundled software but it is ignored by popular anti spyware called Ad Aware , but it can be removed as spyware by some scanners.
Though unlike viruses and worms, spyware does not duplicate itself. These spyware infections are used for commercial gain that is how they are designed. Basic spyware will initiate advertisements like pop ups and they will keep popping up until you install an anti spyware program. Unfortunately spyware is not going away anytime soon so keep your self and your computer protected.