http://dotnet-munesh.blogspot.in/2013/12/cookies.html

Cookies are used for all sorts of purposes, all relating to helping the Web site remember you. For example, a site conducting a poll might use a cookie simply as a Boolean value to indicate whether your browser has already participated in voting so that you don't vote twice. A site that asks you to log on might use a cookie to tell itself that you've already logged on so that you don't have to keep entering your credentials.