The way these 3rd party advertising networks work is they offer the publisher or the blogger (you) a free signup to access links and advertisement codes from a vast pool of advertisers to place on your website. After you have been approved with the network or a certain advertiser, you can then place code on your site pages that generate targeted advertisements. If visitors either look, click, or make a purchase, you can make money.
Cost Per Action or CPA programs are considered effective for making money for bloggers. The most effective way to earn money from a CPA network is for the Blogger to write on a selected product or service and place an advertisement for visitors to purchase that exact product/service. Making money in this manner is profitable especially if there is good quality content that generates lots of traffic from organic search engine rankings. Look at the statistics of a page detailing product information. Is the traffic very targeted? Do a high percentage of visitors coming to that page go on to purchase a product from your selected CPA advertiser? If so, you may be able to enhance your income simply by purchasing more targeted traffic from a PPC network like Adwords.
Cost Per Click or CPC programs involve you getting paid every time someone clicks on your ad. The best and most well known program is Google Adsense. If you have a blog, it is extremely easy to setup and make money. There are other programs available such as Adbrite.com that are worth checking out.