The terms spyware and adware generally refer to computer programs that hide themselves from a user and perform undesirable or even malicious commands without a user's knowledge or informed consent.
Spyware records a user's activities or keystrokes (including logins and passwords) and sends them to a third party, sometimes a hacker. On public computers, spyware may store keystrokes for later retrieval by the hacker. Some spyware programs may also record a user's web surfing history.
In the case of adware, users experience a stream of unexpected pop-up advertising. Adware has also been known to hijack a user's home page or preferred search engine and direct a user to alternate websites.
Spyware and adware can be surreptitiously installed on your PC in a number of ways: