Broadly speaking there are three advertising formats the exist online:

adsense CPC advertising such as Google Adsense, this is effective for sites with a medium size volume of traffic coming through. Earnings are only really significant when you have a high volume of traffic, but even on a small scale it can mean giving site owners a chance to make some commission back, its easy to implement and totally automated.

affiliate CPA, CPL links, this essentially a commission based display advertising model, its real strength is for advertisers looking for an efficent and totally trackable format of advertising that is very efficent. For affiliates its strength is in the increased commissions and ability to be integrated in multiple formats on a site, affiliates also have the advantage of being able to choose which affiliate program to promote.

Display CPM advertising, this is really high volume branding advertising that follows the format of magazine ads, its great for sites with masses and masses of traffic but if your looking for conversions and results its limited.

regardless of the model used, it shows that there are advertising points that can be used efficiently to monetize a site. More often than not it makes sense to use a combination of these in interesting ways to entice potential consumers at different stages of the buying cycle. All this contributes to enhancing the consumers online experience, but go over board and the consumer will feel bombarded and intimidated, so there is an art to integrating advertising on site.