Yes. The user is logging in to Application A. While he is in there, he click a link to Application B. I don't want the user to log in again into Application B. The credentials for App A and App B. Is like implementing a Single Sign On I guess.
You can use Session variables. You will have to navigate those session variables across your web pages. Make sure that the session variable's value changes when the user navigates to application B from application A (Using new session object) and vice-versa.