Google Adwords is an example of a CPC advertising platform.
CPC platforms should have measures in place to stop people from sabotaging your campaign by clicking on the banners multiple times resulting in you being charged for invalid clicks.
Some CPC platforms (e.g. Adwords) require that you bid for your position against your competitors. If you bid too low, your advert may not be shown.
CPC is often considered as cost effective because your offering (e.g. brand / service) gains from advertising exposure anyway at a relatively low cost, depending on the number of click-throughs.