The principle is simple. When you sign up, you download a small program which displays banner ads on a continuous feed. All of these services keep your information private. Home addresses, phone numbers, emails, etc. are NOT distributed to any third parties. Each site has its own privacy policy available for you to read.
Some sites feature encrypted login as well.
Read the FAQs for more detailed info. Referrals are key to making money. In addition to paying you for your surfing time, these companies all pay you for the surfing time of those you refer as well, as well as those they refer, etc. Check out this Profit Calculator to see your potential earnings. Once you sign up for these services, email me and I will help you get referrals.