Sessions CodeIgniter vs sessions PHP

[eluser]WebbHelp[/eluser]
Thanks yoo so much for explane to me, appreciate it very much

[array]
(
'session_id' => random hash,
'ip_address' => 'string - user IP address',
'user_agent' => 'string - user agent data',
'last_activity' => timestamp
)

So:

session_id, is the ID only for the user that visit the site, and with IP and user_agent we can check so it really is him with the ID who is on the computer, only security.
last activity, if I want a session, time limit.

Now to the question; is it like this: Everytime a user visit, they get a session_id that connect them to the data saved for just them. Do you mean that the data being saved in a Database? You mean like MySQL?
If, can I go in with my computer and find for examples session_id (if I have access to be in my MySQL DB).

How, can CI session know when the user has disapper/left the website?

I think this is really interesting and I think you should create a wikiarticle you explane good, so it would be perfect hehe