Spyware is a general term for software that downloads into your computer from the Internet without your permission or knowledge. Sometimes spyware serves you ads, sometimes it collects information about you, sometimes it records every keystroke you type, sometimes it uses your computer as a server. What makes spyware particularly offensive and pernicious, is that spyware breaks into your computer without your permission. This is in a trespassing into your private property or space just as much as someone breaking into your house or business is. And you need to protect yourself.
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