Online merchants have employed a number of different technologies designed to make the shopping experience more pleasant and efficient, with the goal of boosting sales. For example, many online retailers have the option of remembering your credit card information and collect information to get a pretty good idea of what other products you may like to buy. But this typically means you have to give something up in return, usually some amount of privacy.
As a rule of thumb, reveal only that which is required to complete your order. While social media such as Facebook and Twitter have contributed to a culture of "sharers," there are certain details about yourself that no online retailer needs to know. Retailers often try to collect additional details, such as annual household income or favorite types of entertainment, but usually just for marketing purposes. Often it is benign, but sometimes the disclosure of nonessential information can lead to spam, telemarketing calls, or worse.
The most sophisticated online security technology in the world is no match for a compromised password. When you enter a password, you tell the server that you are a certain individual and typically gain access to privileged information. Therefore, choosing a strong password -- one with at least 10 characters, that is difficult to guess, and which has a combination of letters, number, and symbols -- is imperative to protecting your online privacy. Also, do not use the same password for every site.