A cookie, also known as an HTTP cookie, web cookie, or browser cookie, is a string of text sent from a web server to a user's browser that the browser is expected to send back to the web server in subsequent interactions.
- The domain and path define the scope of the cookie - they tell the browser that cookies should only be sent back to the server for the given domain and path.
- Cookies that do not have a specific expiration date and time are automatically deleted when the web browser is next closed. Cookies with a set expiry time are considered persistent cookies, while cookies without set expiry times are considered session cookies.
In online advertising, cookies generally store a unique identifier, and may contain information like what ads were recently seen (so that one user doesn't see the same ad over and over again), when the cookie was created (to discover short duration identities), and other simple attributes.