An HTTP Cookie is a piece of data that is stored on the user's browser. Whenever you make a web-request, all the cookies associated with the domain are sent with the web- request.
On the server, we can access those cookies & read values from them. Hence, cookies help us in maintaining state by storing data on client's machine.
You can add a cookie in ASP.NET in following way:
Response.Cookies.Add(new HttpCookie("Email", "firstname.lastname@example.org"));
To read the cookie, you can use below syntax:
string email = Convert.ToString(Request.Cookies["Email"]);