The moment the user submits their search query, the search engine performs the complex algorithmic calculations on which the ad auction is based. These ads appear when people search for things on the Internet using a search engine like Google, especially when they're doing commercial searches, meaning they're looking for something to buy. For ads to appear alongside results on a search engine (commonly referred to as a search engine result page or SERP), advertisers can't simply pay more to ensure that their ads appear more prominent than their competitors. One of the most common types of PPC is the paid search ad, in which the ad appears when people search for keywords using a search engine such as Google.
They pay more for larger ads and for a more prominent placement, but the effectiveness of those ads can usually only be implied by keeping track of the numbers before and after the sale.