Max's answer is pretty good, but you can earn from them on Hubpages too.
Depending on your topic and keyphrase's used you can earn anything from about 1p to over £50 per click. Advertisers bid on keyphrases that people pump into a search engine such as google, in the hope that the searcher will be interested in their ad and go to their site and buy something. In general the higher the value of the product or service the more advertisers are willing to pay to get customers.
So for instance mortgage companies will pay a lot per click because they can make a huge amount of money from someone who buys their mortgage product - the amount the ad cost (even if it's over £50) will pale into insignificance.
A LOT of publishers go for these extremely high value clicks and so the competition is usually too steep to bother with. I usually go for click values over 50p and up to around £7, with most being around the £1 mark, but for me it's the level of potential traffic that is the most important factor.
The best thing to do is familiarise yourself with google's keyword tool and use the old interface (link on top right of page when you get in there). Put in a keyphrase and then use the drop down menu on the right to choose to see "estimated avg CPC" (cost per click). This gives you an idea of what you might be able to get when someone clicks on your ad if you use that keyphrase (it's actually about 65% of the amount shown.)
You'll need to check how much publisher competition there is for a particular phrase - I use the method described here: http://hubpages.com/business/Long-Tail-Keywords-SE
Then once you've found a good phrase to go for you need to optimize your page which is explained here: http://hubpages.com/hub/increase-page-rank-google-...
It also explains google's keyword tool in more detail, but ignore the part where it says about looking at the green bar (this is a very outdated method of assessing keywords and is not that effective.)
If you're at my house while I'm on hols check out market samurai on the laptop and watch their training videos - very helpful! The link to them is here: http://www.noblesamurai.com/support/
Hope that helps :)