"HTTP cookies, sometimes known as web cookies or just cookies, are parcels of text sent by a server to a web browser and then sent back unchanged by the browser each time it accesses that server. HTTP cookies are used for authenticating, tracking, and maintaining specific information about users, such as site preferences and the contents of their electronic shopping carts. The term "cookie" is derived from "magic cookie," a well-known concept in Unix computing which inspired both the idea and the name of HTTP cookies." – en.wikipedia.org

If you want to store and reuse cookies within .NET you need to use the cookiecontainer. Here is an example on how to work with http cookies under .NET:

HTTP Request :: Store the cookie

wr = WebRequest.Create(uri);
wr.Method = "POST";

HttpWebRequest hwr = (HttpWebRequest)wr;
hwr.CookieContainer = new CookieContainer();

HttpWebResponse httpResponse = (HttpWebResponse)wr.GetResponse();
if (httpResponse.StatusCode == System.Net.HttpStatusCode.OK)
{
     // *** Save the cookies on the persistent object
     if (httpResponse.Cookies.Count > 0)
        this.Cookies = httpResponse.Cookies;
}

HTTP Request :: Use the cookies that have been stored before

wr = WebRequest.Create(uri);
wr.Method = "GET";

((HttpWebRequest)wr).CookieContainer = new CookieContainer();
if (this.Cookies != null &&
   this.Cookies.Count > 0)
      ((HttpWebRequest)wr).CookieContainer.Add(this.Cookies);

HttpWebResponse httpResponse = (HttpWebResponse)wr.GetResponse();
if (httpResponse.StatusCode == System.Net.HttpStatusCode.OK)
{
}