Google AdWords is an incredibly effective way to advertise, utilising context to display a message.  In other words, Google reads what is on a webpage, then works out ads that may be relevant to that topic, and displays them for you on that page.
It’s not perfect – for example sometimes websites or news items that discuss airline crashes are served up with ads for Qantas, but on the whole it gets the ad right – it’s far more effective than traditional advertising placement.
The Age has a beginner’s guide here, it’s pretty solid:
Beginner’s guide to Google Adwords – BizTech – Technology – theage.com.au.