When you go online and identify yourself to a website, whether it's through logging in, entering credit-card details to make a purchase, or just allowing the site to read the email address you've typed into your browser preferences, you are in effect allowing the website to record data about you and your preferences. Most sites will treat this data sensibly, making sure that it is only used within the company or for the purpose you surrendered the information for originally, and will ask your permission to use it for other purposes. However, there are less scrupulous sites around, that may distribute your data indiscriminately. Be aware that this can happen and try to protect your personal data carefully - it's a valuable asset. More of this in the next chapter.