This portrait helps companies target content at specific markets and consumers, helps employers look into your background, and helps advertisers track your movements across multiple websites. Whatever you do online, you might be leaving digital footprints behind. (http://www.internetsociety.org/your-digital-footprint-matters)
|A diagram of how your internet works|
The above is a very simple yet accurate depiction of how you access the internet. The ISP, your (Internet Service Provider) provides this service to your home modem/router, which is then accessed by your devices. Think of it this way. Every site you want to visit is sent as a request to your ISP, and then is relayed back to you to access. The ISP can simply make a database of these requests of information from you, and then plot it to see what you’ve been upto lately. You aren’t as hidden as you think you are.
There’s several ways you leave Digital Footprints. (From internetsociety.org):
Websites And Online Shopping: Retailers and product review sites often leave cookies on your system which can track your movement from site-to-site, allowing targeted advertisements that can show you products you’ve been recently reading about or looking at online.
Social Media: All those +1s, Retweets, and Facebook comments (even private ones) leave a record. Make sure you know what the default privacy settings are for your social media accounts, and keep an eye on them. Sites often introduce new policies and settings that increase the visibility of your data. They may rely on you just clicking “OK” to whatever terms they are introducing, without reading them.
Mobile Phones, Tablets, or Laptops: Some websites will build a list of different devices you have used to visit those sites. While this can often be used as a way to help secure your account, it is important to understand the information being collected about your habits.
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I STRONGLY RECOMMEND YOU WATCH THIS.