Cookies are small pieces of data stored on a user's device by a website. They are used to store information about a user's activity on the website, such as their login information or their browsing history. Cookies can be either first-party, created by the website the user is visiting, or third-party, created by a domain other than the one the user is visiting. There are several types of cookies, including session cookies and persistent cookies. Session cookies are temporary and are deleted when the user closes their browser, while persistent cookies remain on the user's device until they expire or are deleted. Cookies can be helpful for website functionality, such as keeping a user logged in or remembering their preferences. However, they can also be used for tracking and advertising purposes.