Zend Session is a component made by Zend which helps in managing and preserving sessions in any application which uses Zend Libraries.
Session is a one-to-one relation between user agent (a browser) and the server-sided persistent data.
Zend_Session helps manage and preserve session data, a logical complement of cookie data, across multiple page requests by the same client. Unlike cookie data, session data are not stored on the client side and are only shared with the client when server-side source code voluntarily makes the data available in response to a client request. For the purposes of this component and documentation, the term "session data" refers to the server-side data stored in » $_SESSION, managed by Zend_Session, and individually manipulated by Zend_Session_Namespace accessor objects.
Zend_Session_Namespace instances are accessor objects for namespaced slices of $_SESSION as per Zend Documentation