determining if the user is logged in

I am new to wordpress, and trying to get my head around the idea of sessions and all. I read in a lot of places that session concept is not used in wordpress. Anyway, I downloaded the my-login-theme plugin. My question is, when a user navigates to some secured area, I want to check if that user is logged in or not. Once I check that, I want to get user details like email, and full name. Is this possible in wordpress?

How to hide media uploads by other users in the Media menu?

Simply put – how might I completely hide media files uploaded by any other users other then themselves? Id like to disable access to these links & files since they are a way for non-admin users to be able to access and edit a custom post type that is normally not visible to them otherwise in the backend of wordpress.