Do I pay for every click on my ad?

You  need to pay for every click in CPC(Cost Per Click) advertising on your AD except in the following scenario

a) If the same visitor who has clicked previously is clicking again normally you wont be charged again. But remember the IP address of the visitor is the unique key.
b) If the visitor click accidentally on your ad and he closes your website immediately, normally those clicks would not be charged.
c) If you can prove to google/yahoo by providing your website logs that the following clicks are click frauds normally they wont be charging you.