Banner advertising on this site is provided by Google Adwords. With AdWords cost-per-click pricing, it's easy to control costs - and you only pay when people click on your ad.
With the Adwords system you sign up with Google and buy keywords (eg "new zealand mountain bike rides"). Google then shows your ad on searches that match those keywords *and* on any web page (on an affiliated web site like this one) that matches to those words.
This has the benefit for you that your ad is seen by those who are directly interested in your keywords, and also across a range of relevant web sites. [The benefit for me is they do the admin a lot better than I could.]
Apply Online: Create ads and start managing your account - takes just minutes. Learn more from Google