I've been wondering this for a while now.
What are the differences between Joomla sessions and php sessions? Are they interchangeable?
JSession is based on the standard PHP session handling mechanism but I would not say they are interchangeable, as Joomla is directly "talking" to JSession and not to the super global.
In backend you select a session handler (the default being the database) where Joomla will store session related informations, basically it's serializing the content of
When a cookie is sent, no actual readable data is stored, just a session identifier.
It also provides a very simple, namespaced interface to store and retrieve data from the user session. Read an entire article about this on ostraining.