session_start() question

Can someone explain session_start() as I am still don't have a full grasp of sessions. In the following example I have to use session_start() on both of the pages below, but once a session is started why would I need to do this - does it not carry from page to page? - confused. page-a.php example: session_start(}; if($_SESSION['loggedin']) { echo "You are already logged in"; } else { *** the login form appears here *** <FORM NAME="Login" ACTION="page-b.php" METHOD="post"> .....etc.... } page-b.php example; *** code to validate input *** if input is correct.... session_start(); $_SESSION['loggedin'] = "True"; $_SESSION['email'] = $row['email']; $_SESSION['names'] = $row['firstname']." ".$row['lastname'];