That varries from network to network. keyword relevance is one across the board, sometimes previous track record for ads clicked is taken into account, as well as the look on the site. just like
SEO, a page with good content, say around 600 words at least will get a better ranking than a page full of banner ads, or a opt in page. also, things like a privacy policy, and outbound links will be taken into account.
if you have a page with only affiliate links, or a page with at least one link to a valuable blog or website, the page with the links to free content will get better placement.
as far as keywords, there are a lot of factors that go into the value of a keyword. # of competing ads, average bidding price, and search volumes are just a few.
just like
seo, the only people who know all the factors involved are so bound up in non-disclosure agreements that they aren't talking.
Now, I am no expert on PPC. however, I can say be careful. you can spend a lot of money with little to no income before you work out a good campaign. Most of the people I know who are successful with it say to expect to loose about $500-$1000 before you start seeing a campaign work.
It takes constant testing and tweaking to get it right, even with experience under your belt. Some see success faster, but i have not seen anyone hit it perfect with their leap into a new campaign.