What's the difference between CraftSessionId and a normal PHP session? Doing some testing, and stumbled across the "CraftSessionId" cookie...

What's the difference between CraftSessionId and a normal PHP session? Is there any difference at all? What's the best way to refer to the current session?
They are one in the same.

Craft simply renames it to "CraftSessionId" rather than the default "PHPSESSIONID".

The only additional things we do to it is set the "httpOnly" flag so that JavaScript can't read it and change the place it is saved to by default (varies per server) to craft/storage/runtime/sessions.

You can see that in action in HttpSessionService, which extends Yii's CHttpSession, which is a convenient wrapper for PHP's session functionality.