[TYPO3] Altering content based on if user is logged in

[Stephen Bungert]

The site I'm working on should have a log-in system for members. When 
logged in they see things that unlogged visitors don't see. If I was 
doing this in a non typo3 site I could just use php to check the 
username and password in a database and render the page based on if the 
user is logged in. The first thing I would do would be to redirect the 
user to the same page,

where the post variables would be checked, and if ok, a session started 
and the user would be logged in.

How does one do this with a TYPO3 based website?