For ages webmasters used to have sites with affiliate banners on them, and wait patiently for a user to sign up to a service they had advertised on their site before they could receive their commission.
That was, at least until Google came around and made the job of a webmaster a little bit more interesting. They branded the pay per click idea worldwide and got advertisers to part with their cash by bidding on keywords that would be entered into their search engines, or by having sites that was relevant to that service. Cashing in on this innovation where webmasters, and would be webmasters, (well basically anyone who could buy a domain and grab a template).
Websites everywhere started showing up, aggressively marketing themselves just so that, a user would click on one of their adverts. Google only paid for the click and not a percentage of the sale, like the more traditional affiliate sites.
The same practice carries on till this day. Webmasters try and predict future trends by driving traffic to their sites in hope that users will click through on their adverts.
Having pay per click adverts on your site is as easy as signing up to a whole host of companies, Google being the most popular. Get your website accepted, and complete a payment method. Payment can vary from bank transfers to a cheque sent to you in your name.
Just note that under normal terms and conditions you would have to choose one advertiser for a site page, for providing pay per click adverts. So make sure before you sign up if you intend to have more than one on any given page.
Popular Pay per Click sites are: