You can earn money on hubpages through Google Adsense, Amazon, Kontera and ebay. Most writers on this site earn money through Google Adsense and Amazon.
(1) Google Adsense.
In short, when you publish an article (a hub) on this site, Google places its ads on your page. If visitors to your page click on these ads, Google pays you the amount arranged with advertisers. Most clicks are worth around 2 cents and you need to reach the treshold of a 100$ for your first payout. Hubpages take their provision because you are using their space to post your writings and earn money. This is a very short answer - it takes a hub to make it all clear so I suggest to read the help section and hubs by different hubbers on this topic (type earning money on Hubpages in the search box above).
You can promote amazon products on your pages. If buyers order an item through your link you recieve a percentage. Again, read different hubs on this topic. Earning money online is not easy - you need to learn a lot, apply your knowledge and be patient to see the results.