PHP sessions are used to store user information and pass that information to a group of pages. But where is the session information stored, on the user computer or the web server?

Suppose I logged into a site. Now that site has my information stored in session. If another user logs in from another computer with his own username, does his information gets stored in a session and mine is removed or is there a array of sessions? I just dont understand the idea of session.
If you didnt get what I am asking, we have same session variable for everyone. like $_SESSION['username'] is used for everyone in a site. If I log in my username is stored in that variable. If another user logs in, does his username replaces mine or is there a sort of array which stores all usernames.

Hope you understood what I am asking.

Any help would be appreciated.