an alphabetical list of words relating to a specific subject, text, or dialect.
This helps an advertiser to target websites or placements basis the topics or interests on which one thinks could be the best match for the brand in order to create awareness about the brand or can be used to generate an end goal which is associated with the campaign objective.
Interests/topic/Affinity basically targets a website contextually wherein Google matches the content of the webpage of the website and determines the theme of the same in order to give the advertiser the flexibility to target the website basis the theme or topic they think is ideal for the website. Though the websites can be contextually targeted wherein an advertiser can target certain keywords to give the ad group a theme through the best practises is to use a combination of both (topic & keywords) to target websites which are related to the defined theme of the ad group and campaign.
This targeting approach is a less granular method of targeting compared to the traditional way of using keyword targeting.