A click-to-donate site is a website where users can click a button to generate a donation for a charity or cause. The money for the donation comes from advertisers whose banners are displayed each time a user clicks the button. While not directly contributing (though many sites offer additional ways of support), visitors are making a difference in the sense that, had they not visited, no donation would have been given.
In most cases, the donation generated by each user only amounts to a few cents, but the goal is to accumulate enough clicks to add up to a significant amount.
Many charities launched this style of program in the late 1990s. However, the constriction of online advertising spending around 2001 following the dot-com collapse caused many sites to be closed. Yet there are still many which are in operation notably FreeRice, The Hunger Site and Feedeveryone.ml.