Well you know when you do a search in google and you get a lot of small box advertisements on the right hand side of the page? Well, they are pay-per-click ads! Web site owners have to pay a fee for each click on those ads, the higher the price they are willing to pay for each click, the higher up they appear on the search results page.
For instance, if a webmaster bids £0.05 on the term 'celtic ring'. If no other webmaster bids more than this, he will stay in 1st position. If a competitor bids £0.06, then the competitor moves into to the coveted 1st position. Thus the bidding war begins and you may find some keywords costing as much as £5.00 per click!
This form of web marketing can be highly benificial but be careful on how you want to spend your budget. Luckily, Googles AdWords offers an option where the user can select a pre-determined monthly budget, once you reach that budget your ad is put on hold until the end of that month. If you want real targeted advertising this is my uttermost recommendation. One vital warning though; choose your keywords wisely!