All of the major search engines offer PPC advertising. Your advert will appear for the phrases (or keywords) which you have selected. For example, if you sell van insurance you might want to bid on terms such as van insurance, cheap van insurance, buy van insurance, van insurance quote etc The position in which your advert appears is determined by a number of factors including:

Your Bid

The price you have said you are willing to pay for a click

Relevancy

This is determined by a number of factors including - how well the advert you display relates to the keyword which you're bidding on and how well the web page which your visitor will land on relates to the keyword

Click Through Rate

Click through rate is expressed as a percentage, and is calculated by dividing the number of users who click on your advert by the number of users who see it. This allows the search engine to determine how relevant searchers find your advert as typically searchers will only click on an advert which they deem appropriate to their search PPC text