A paid to click (ptc) site is exactly what is sounds like. You get paid to click a link and visit a website for about 15 to 30 seconds.
The ptc company then pays you a very small percentage to visit these sites. It's a win win situation, the website generates more traffic and you earn money for visiting. Do not get me wrong you are not going to make a fortune from a ptc site. This is however the easiest and the only free way to make money online that I have come across. Most of these sites pay you 1 cent for every link that you click. You can also refer people and make money on all of their clicks.
Here is a brief illustration of how building a downline can multiply your efforts.
Earning example (for one site only):
You click 10 ads per day = $0.10
You have 20 referrals who click 10 ads per day = $2.00
Your daily earnings = $2.10
Your weekly earnings = $14.70
Your monthly earnings = $63.00Sign Up