Another way to describe it is... Let's say you want to buy traffic from me. Traffic meaning visitors coming to your website or page. If I send you 100 visitors to your website, I just sent you 100 clicks. To get those clicks you have to pay a set price per visitor. That is pay per click advertising.
PPC stands for pay-per-click, a model of internet marketing in which advertisers pay a fee each time one of their ads is clicked. Essentially, it's a way of buying visits to your site, rather than attempting to “earn” those visits organically.