Sessions CodeIgniter vs sessions PHP

[eluser]GSV Sleeper Service[/eluser]
CI Sessions do not use $_SESSION, the best way to understand is to look in system/libraries/Session.php, where'll you'll notice that CI have essentially re-invented the wheel, be warned that if you decide to use the CI Session library you won't be able to store more than 4KB of session data.
My advice is to check the wiki and look for 'native session library'