Application variable which remain common for the whole application (your website). Application variable’s value used in whole application. Application state allows you to store global objects that can be accessed by any client. The example of the application variable is site user count.
Session variable which are common for whole application but for particular user. Session is maintain in the page means you can create new session and use to. Example of this when user is login into the site at that time we create one session called user. After the log off we kill this session.