Spyware is software that gathers information about a user, usually without their knowledge or informed consent, and then passes this data to others.

As with computer viruses it comes in various forms but, in contrast, much of what is commonly referred to as spyware is currently perfectly legal.

The information it gathers is used by companies to target users with tailored advertising, interrupt their online experience with annoying pop-up windows and bombard them with unwanted emails promoting products and services.

While most of us wouldn't worry too much about something that tracks the websites we use, other spyware can record highly sensitive personal information such as passwords and credit card numbers.

This is far more worrying, especially for the millions of us who use online shopping and banking services.

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