There are various ways to target display ads which include the following:
Contextual: The types of content users consume.
Placement: The types of websites users browse.
Remarketing: Target users who have visited your website before to attract them back.
Interest: Target based on online behaviors and sites users browse.
Topic: Target based on the topic of websites for more relevancy.
Location/Language: Show ads based on where users are in the world and also what language they speak.
Demographic: Target users based on their age, gender, income, and more!