Pay-per-click advertising, also known as contextual advertising is one of the most common & work-free ways of monetizing a website. You’ve probably seen contextual advertising pretty regularly without realizing what they were.
The way the system works is that, you, the website owner will sign up to an advertising network (the most common one being Google Adsense). You then designate areas of your website to be used for their ads, and they place ads that pertain to the content of yoru site.
For example, if your site is about potty training, then the advertising networks will place ads pertaining to early childcare, childhood, parenting, or anything that a parent of a young child might be interested in. If a user clicks on the link, you get paid a certain amount of money.
The amount can vary a lot depending on what your keywords are, and can range between a few cents to over $40.00 a click. Note, the majority of clicks will range between $1.10 & $2.00. The higher page-rank you site has, and more traffic the higher the pay-per-click rate.
This segment will concentrate on Adsense, how to apply and set it up, as well as things to look out for & resources for further learning. There are other ad networks, but AdSense is the most acclaimed and common. A list of other ad networks can be found in the resource area.