There are two basic ways:
1) Cookies. These are stored on the client computer, so even if he comes back a week later, you can retrieve the info. http://msdn.microsoft.com/en-us/library/vstudio/ms178194(v=vs.100).aspx
2) Session. These are stored on the server, and will be lost when the browser is closed, or after about 20 minutes of inactivity. http://msdn.microsoft.com/en-us/library/system.web.httpcontext.session(v=vs.110).aspx
There are other ways involving databases, but unless you force your users to log in, that is probably not what you want to do.