Session problems

Hi there, I am trying to get my head around Sessions but not getting very far. I can't seem to work out exactly how you keep track of the variables. If you register a variable at the start of the page, will it then track this variable and take it from page to page by its self, or do you have to do something in the code to tell the session its changed, like register it again? I have got to grips with most things so far, but sessions are fighting back, are there any good tutorials on the web you can recommend? I have already used web monkey for most things, but their session section was a bit crappy. Thanks