What is PTC website?

-> Paid to Click (but what?)
Probably you already know what is a PTC (sometimes called PTR - Paid to Read Site) site. If you don't here is a definition you will find in many PTC sites:

"At ---.---, you get paid to click on ads and visit websites. The process is easy! You simply click a link and view a website for 30 seconds to earn money. You can earn even more by referring friends. You'll get paid $0.01 for each website you personally view and $0.01 for each website your referrals view. Payment requests can be made every day and are processed through AlertPay/Paypal. The minimum payout is $10.00."

(default description from the bux.to clone script)

A very simple definition: click some links (ads), earn money.

PTC sites are "make money online" programs. A PTC can be considered as a low risk program because it is not required to invest. But if you are not careful you can lose, of course, lots of money.