Spyware is software that is added to your computer without your knowledge. As the name implies ... Its purpose is to spy on you and gather information -- sending the information it collects to others to use in whatever way they want -- without your permission.
Spyware can collect almost any type of data like Internet surfing habits, user logins and bank/credit account credentials. Some spyware can change your computer -- installing software, changing settings, redirecting browsers and slowing your Internet connection.
This kind of software is mostly used for serving pop-up advertisements and tracking/storing Internet users' movements on the web.