Paid to Click sites are introduced here.
These kind of sites are generally requires you to click advertisements and offer you minimum $0,001+ for clicking and watching an advertisement. Investment is not mandatory in such kind of sites, but some of them may offer you more money by investing into their sites. So, some of them may have some ponzi elements in it.
Although most of the PTC sites are free money, be cautious when you make money online if a site requires you to invest into their programs.