Major online ad networks offer different ways to target customers. A brand that sells running shirts for women might turn to Google to find people who the company believes are female and interested in running based on their search and browsing history. The company may turn to Facebook for people in a women’s running group.

Many of Amazon’s features are similar to those of Google or Facebook, like offering ways to target users based on their interests, searches and demographics. But Amazon’s ad system can also remove a lot of the guesswork by showing ads to people who have bought the shirts on Amazon.com.

Advertisers have long run some targeted campaigns through Amazon’s ad network. Many have done that by working directly with Amazon’s staff, who would place their orders on their behalf. That option has historically been focused on larger brands because it requires a minimum advertising commitment. Over time, Amazon has given more advertisers and their agencies access to the self-service system to run their own targeting campaigns on and off Amazon’s websites, and at a variety of spending levels.

Users of the self-service system can choose from hundreds of automated audience segments. Some of Amazon’s targeting capabilities are dependent on shopping behaviors, such as “International Market Grocery Shopper” and people who have bought “Acne Treatments” in the past month, or household demographics, such as “Presence of children aged 4-6.” Others are based on the media people consume on Amazon, such as “Denzel Washington Fans” or people who have recently streamed fitness and exercise videos on Amazon. The company declined to comment.

Just the Cheese, a brand run by Specialty Cheese Company in Reeseville, Wis., makes crunchy dried cheese bars that have taken off as a low-carb snack. By using algorithms to analyze how Just the Cheese’s search ads performed on Amazon’s site, the ad agency Quartile Digital noticed that people who searched for keto snacks and cauliflower pizza crust, both low-carb diet trends, also bought a lot of cheese bars. So Quartile ran display ads across the web targeting Amazon customers who had bought those two specific product categories. Over three months, Amazon showed the ads on websites more than six million times, which resulted in almost 22,000 clicks and more than 4,000 orders.

That 20 percent conversion rate — a sale to one out of five people who clicked on the ads — was “amazing,” Mr. Knijnik said. “That is the kind of powerful granularity for building the target audiences that just Amazon can give you.”

Like other ad networks, Amazon uses cookies and other technical tools to track customers from its site onto other websites. They let the company know that a person who recently bought a diet book is now reading news on CNN and could be targeted on that site with an ad for a protein bar. Amazon does not tell the advertisers who that user is, but it does serve her ads on the brand’s behalf.