An adware is any software that plays, displays or downloads advertisements to a computer automatically. Pop-ups usually take the form of these advertisements so as to generate income for the advertiser. Generally speaking, adware does not pose any danger. However, some adware comes with integrated spyware that are invasive to privacy.
Adwares are designed to record the sites which the users visit and present them with advertisements or types of goods that are similar to the sites frequently visited. It is way wherein the developers cover their developmental costs; they may even allow users to use the program for free or at a special discounted price. The income generated in advertisements can be used for further development of the software but some users may see these adware as interruptions from their undertakings.
Not all adware are used just for advertising, some are also sharewares. They are sharewares because one has the option to use it as an adware that pops-up advertisements or pay for the registered version and use the program without the interruption of ads. An example of a shareware is a certain e-mail client. Here, users are given a free trial period which all features are available. After such time, the user will be given options: to use the shareware for free but with limited functionality, use it with full function but it will display advertisements or a paid mode that allows the user to use it with full functions but without the pop-ups.
Some adwares can also be classified as malwares (malicious software) or spywares because they steal information from the users. Once, a certain corrupted many computers by forcing it to display advertisements that were obscene and sent browsing details to other parties.
It is very important to remove these because as mentioned, some will include spywares and most of them run without the user realizing it. They can also dramatically slow down the performance of the computer. Many anti-virus programs have been written that include the detection and removal of adwares, almost all of them in fact. Some may offer a separate adware and spyware removal package. There are also very good anti-virus programs that are free to download which already includes adware and spyware removal.