What is Contextual Advertising?

I have referred to BidVertiser and Google AdSense/Adwords as being examples of Contextual Advertising. Another example is the Yahoo Publisher Network (which is currently only accepting sites that have a predominately US & English base). So, what is it?

Contextual advertising is advertising that is dynamic and changes to reflect the content of the page it's displayed on. For example, on a page where the primary content is about mobile/cellular phones, the advertising displayed will typically be for mobile/cellular products. e.g. ringtones, screensavers, wallpaper, or for phones themselves.

This should result in a higher clickthrough ratio than traditional banner advertising, because surfers are (hopefully, it doesn't always work quite right) only viewing ads specifically targetted at their interest. If they are viewing a page on mobile phones, then it's supposed they are interested in mobile phone products.

BidVertiser

BidVertiser is another contextual advertising program, similar to Google AdSense/AdWords where you get paid from clickthroughs of ads placed on your site. It also includes a referral program, where you can earn from referring publishers or advertisers to the program.

Minimum payout threshold is $10 through PayPal, or $50 by cheque. I haven't yet used this program anywhere, as you can't use a program similar to AdSense on any site running it (I checked, Google's ToS seemed to only restrict it by page). I'll decide which site to use it on, or maybe create a new one.