"The term 'spyware' is not well defined. Generally it is used to refer to any software that is downloaded onto a person's computer without their knowledge. Spyware may collect information about a computer user's activities and transmit that information to someone else. It may change computer settings, or cause 'pop-up' advertisements to appear (in that context, it is called "adware"). Spyware may redirect a Web browser to a site different from what the user intended to visit, or change the user's home page. A type of spyware called 'keylogging' software records individual keystrokes, even if the author modifies or deletes what was written, or if the characters do not appear on the monitor. Thus, passwords, credit card numbers, and other personally identifiable information may be captured and relayed to unauthorized recipients. Some of these software programs have legitimate applications the computer user wants. They obtain the moniker 'spyware' when they are installed surreptitiously, or perform additional functions of which the user is unaware. Users typically do not realize that spyware is on their computer. They may have unknowingly downloaded it from the Internet by clicking within a website, or it might have been included in an attachment to an electronic mail message or embedded in other software."
CRS Report for Congress, RL32706