where is the logged in user showed?

Hi!
I am developing a live-feed for WP/BuddyPress.
I am an Php-coder, but quite new to WP.
To start with, I need to know how to catch the ID of the logged-in user.
Anyone who knows how to do that? Probably! I guess that I get an answer on that quicker here, than I find it out myself.

So, let’s say, I am building a widget that shows the logged in user(I know how to do the base for a widget), how do I call, to show the logged in user? Doesn’t matter if I get the name, mail, ID, or whatever, I just need an identifyer. Hope 4 help!
/Johan