Adware and spyware are malicious programs, protocols, scripts, or other software designed to collect personal information, facilitate cybercrime, or just provide a nuisance for the targeted user. Both adware and spyware fall under the larger umbrella term of malware, which includes everything from annoying bloatware to dangerous Trojan horse applications.
While both terms are often used interchangeably, they are technically different. Adware refers to software that generates advertisements within the user interface of another piece of software or system, sometimes installed without the user's knowledge or consent through a backdoor channel like a virus or rootkit, sometimes through greyware like being hidden within a product's Terms of Service. Adware can be malware, but frequently its dangers are limited to the realm of nuisance rather than active threat.
Spyware, however, does imply malicious intent. Spyware is a piece of software, a script, or some other system designed with the explicit intention of collecting personal and/or financial information on a user. This information can then be used for advertising or marketing purposes, cybercrime like cyberextortion or identity theft, or to gain control of the infected device.
Some of the most common and notorious forms of spyware include traffic redirect packets like CoolWebSearch and the infamous Bonzi Buddy, which exploit browser weaknesses to redirect users to websites with pop-up ads and other revenue generating schemes. Zango, 180 Solutions, and other such "stealware" companies illegally modify HTTP affiliate requests for advertisements and prevent you from seeing ads or even visiting the websites of their competitors. Trojan horse applications like Zlob and DarkComet install themselves on a device in the guise of being another executable, and relay information back to the host device. Keyloggers are a similar piece of spyware, literally tracking everything the infected user's device types and relaying the information back.
Protecting yourself against adware, spyware, and general malware is critical. Ensuring that your computer is equipped with a solid, reputable anti-virus software is the most important thing you can do to prevent infecting your device, along with practicing good cyber-security.
Here are some warning signs that your device may be infected: