Adware and Spyware the same difference
Adware is the cousin of spyware. People are made to believe that there is a difference between the two by giving it another name. Adware is mainly used as a marketing tool to monitor people's behaviour on the Internet, to determine which products they are interested in. Spyware reports these statistics to its creators while adware supposedly doesn't. Adware is sometimes the carrier (Trojan Horses) of spyware and viruses and is hard to get rid off. Anti-spyware software also removes adware, for one simple reason, it falls under the same category as spyware and has the same characteristics as spyware. Some adware programs are bundled with free software packages and when you remove the adware from these packages, you render the software useless. Rendering the software useless is a small price to pay compared to the threat posed by the adware. Software that needs adware to operate can't be that legitimate after all.
Other adware programs breach your system through browser vulnerabilities and certain web sites exploit security holes to install adware on your system. Rootkits are used to hide adware from the user and backup programs (adware guards) reload the adware into the memory if you try to close them. Spyware uses exactly the same techniques to infiltrate and maintain itself on your system. Another characteristic of adware is the consumption of system resources, also common among spyware. Adware is labelled as legitimate software. How can you call it legitimate software if they hide themselves from the user, refuses to leave your system and keep all the system resources for themselves? You never know what they are busy doing behind the scenes. Legitimacy and transparency goes hand in hand when it comes to software. Software that is not transparent in its actions and the information it collects can never be called legitimate software.
For further reading, refer to our article Why Distinguish Between Spyware And Adware?