The Paid To's

PTC- paid to click
This is where you get paid to click ads- advertisers pay money for their ad to be shown and most of the money goes back to the members.

PTR (1)- paid to read
This is similar to paid to click except there is a piece of text normally about the site from the advertiser before you go on to the actual ad.

PTR (2)- paid to read
This is also similar to paid to click except you get it through an email.

PTSU- paid to sign up
Advertisers pay you to join their sites- the requirements differ from each advertiser and each site.

PTS- paid to surf
This type normally pays much less than paid to clicks but otherwise they have the same principal. Because they cost less, there are normally lot's of them as the advertisers normally find this a good deal.