A cookie is a small piece of data that is stored on a computer or mobile device by a website or app. It is used to identify the user and track their activity as they interact with the website or app.

Cookies are often used to remember user preferences, such as language or font size, or to keep track of items in an online shopping cart. They can also be used to personalize a user's experience by showing them content that is relevant to their interests or location.

Cookies are typically stored in a user's browser and are sent back to the server each time the user visits the website or app. This allows the website or app to recognize the user and provide a customized experience.

There are different types of cookies, including session cookies, which are temporary and only last for the duration of the user's visit to the website or app, and persistent cookies, which are stored on the user's device for a longer period of time. There are also third-party cookies, which are set by a domain other than the one the user is visiting, and can be used for a variety of purposes, including tracking user activity.

In the context of display advertising, cookies are often used by advertisers and ad networks to show targeted ads to users based on their interests and online activity. For example, if a user visits a website about cooking, an advertiser might place a cookie on their device that is associated with the cooking content. When the user visits other websites, the advertiser could then show them ads for cooking-related products or services.

Cookies can also be used to track a user's activity across different websites and devices, allowing advertisers to build a profile of the user and show them more relevant ads. This can be done through the use of third-party cookies, which are set by a domain other than the one the user is visiting and can be used to track user activity across different websites and devices.

While cookies can be useful for showing targeted ads, they can also raise privacy concerns, as they involve the collection and use of personal data. To address these concerns, many countries have enacted laws and regulations(e.g. GDPR and CCPA ) that govern the collection and use of personal data for ad targeting, and many companies have adopted privacy policies and practices to help protect user privacy and provide greater transparency about their data collection and use practices.