There are a few different types of GPT websites, some of them are direced towards surveys, some clicking, and some reading.
Some websites offer more than one type of making money, but many are geared towards one main theme.
Survey Websites
Often referred to as just "GPT websites" these website's main way of making money is by completing surveys. They can vary in length and what it asks of you, the pay for each also usually varies depending on what you need to do.
Clicking Websites
These websites, known as "PTC" websites, pay you for each ad you click. Often times you will need to stay on the advertisement you clicked for 20-30 seconds for the offer to be counted as viewed. Most PTC websites will have a small counter that counts down how long you must remain on the advert that you are currently viewing and will tell you once you've been credited.
Reading Websites
These websites pay you to read e-mails and other things, they usually also offer some other form of making money like surveys. Usually referred to as "PTR" websites.