The web has totally transformed advertising. In the past a TV ad or poster campaign was difficult to judge in terms of success. For example ACME advertise a choclate bar, sales go up 50%... However there was no sure way to say that had they spent less or more money on advertising they would have got the different results.
However with the web it's possible to track the success or failure of a web advert because there is a direct link between an advert and the the number of people who click on it.
There are generally 2 models in opperation, CPC (cost per click) and affiliate commision rates.
With CPC when somebody clicks on an Advert the advertiser pays a certain price, normally in the pence range. With commision rates payment is only made when somebody then subsequently goes on to buy something or performs a certain action on the site, signing up for example.
CPC is only really an option if your site already has a high level of traffic as advertisers generally want a set level of page impresssions (number of times an advert is shown)
Anybody can get in to the commision based stuff, for example with Amazon, the problem with it of course is getting people to buy items from the site. One popular method is google ad words which can be pretty good but there are plenty of rules which mean people might not be too keen to click on the ads.
https://www.google.com/adsense/?sourceid=aso&subid=ww-en_GB-et-awhome&medium=link&hl=en_GB
http://affiliate-program.amazon.co.uk/gp/assoc iates/join/main.html
As you say the key to success is a decent and well visited website.