Сессионные куки
Webpages have no memories. A user going from page to page will be treated by the
website as a completely new visitor. Session cookies enable the website you are visiting to
keep track of your movement from page to page so you don't get asked for the same
information you've already given to the site.
Сессионные файлы cookie в ASP.Net
When a user connects to an ASP.NET application, a unique session ID will be affiliated
with the user. If nothing is put in the session however, no cookie will be sent to the
browser. This means that the user will get a new session ID the next time a new url is open or
the page is refreshed. If something is put on the session (HttpContext.Current.Session["Hello]
= "hello") however, ASP.NET will issue a cookie called ASP.NET_SessionId. This cookie contains
the user's session ID and the cookie will expire at the end of the session (when you close
your browser).
Подробнее см.