Companies submit their site's descriptions and titles, along with a list of keywords to the engine.
They also specify the amount of money that they're willing to spend on each keyword. Depending on how much money others paid, rankings for a particular keyword maybe very high or low. Most engines specify a minimum amount of money needed for a company to open an account. Many think that paid rankings will decrease the relevance of search results by allowing any site with money to rank high for any keyword. Relevance doesn't guarantee the quality of the site.