There are three main program types to earn money clicking in PTC (paid to click):

• PTC
• PTR
• GPT

PTC:
In PTC sites (paid to click) we are paid to click and see advertisements during a pre-determined time period.
See here the best PTCs in the market.

PTR:
In PTR sites (paid to read) we are paid to click in links (received on email or site inbox) and usualy do searches. Almost every PTR sites are PTS sites (paid to search).
See here the best PTRs in the market.

GPT:
In GPT sites (get paid to) we are paid to do some action: view ads, do searches, register in another sites, or buy something. In that way PTC and PTR sites are two types of GPT sites.
See here the best GPTs in the market.

In PTC programs the earnings are, aparently, bigger than in PTR, because the advertisments value are bigger (generally between 0.5-1 cents at PTCs and between 0.25-0.5 cents in PTRs). However we cannot look only at ads value. Minimum payout are higher in PTCs, and many PTCs sites are not trustfull. There are several PTR websites which have 2 or more years. I don't know any PTC paying on time with more than 6 months.

Several PTCs closed their websites or jus continue withou making payments to the users. Their are known as SCAM.

PTR sites are more boring to the users because they have to do searches.

Resuming: PTC sites can generate more earnings (if they pay) but PTR sites are more trustfull.

Another important concept are referrals and refferral comission. We earn everytime we click but also when when our referrals click on advertisements. Referral comission varies from site to site (usually between 5-10% in PTRs and between 50-100% in PTCs).

We earn a referral everytime someone register with our link. In everysite there is a link that we can use to redirect people to register in our referral list (downline). This link can be used directly or in banners.