Pay Per Click is a method of advertising and publishing such advertising on the Web.
The best example is Google's Adsense and Adwords combination.
Adwords is the advertising program, where Google offers vendors of products and services a way to advertise on relevant sites, and when someone searches for related topics on Google.
Adsense is the flip side, a network of web sites where the advertisers' ads appear.
Google is a go-between.
In general clicks cost, for the Adwords advertisers, range from $0.03 to as high as one is willing.
The income for the Adsense publishers ranges from $0.01 to $20+.
You would display an special script on your site which would display the best fitting advertising related to your web page's topic. This program is called the Adsense.
When someone clicks on such an ad, you get a fraction what the advertiser paid to Google through Adwords.