twentyten footer specifics

Can anyone help explain the specific parameters used in the twentyten footer? Here’s what I understand..and don’t

do_action( ‘twentyten_credits’ );
do_action is an action hook which is fired when an event happens, but what does this hook do and what is the event? I don’t see ‘twentyten_credits’ in the functions.php file.

php echo esc_url( __(‘http://wordpress.org/’, ‘twentyten’) );
esc_url cleans a url, but what does the second param of ‘twentyten’ mean?

esc_attr_e(‘Semantic Personal Publishing Platform’, ‘twentyten’);
Same question, esc_attr also cleans a string, but what is the second param ‘twentyten’?

printf( __(‘Proudly powered by %s.’, ‘twentyten’), ‘WordPress’ );
prints the first param, but what does ‘twentyten’ mean in second param

I feel like I am missing something obvious, but when I looked each function up in references the second parameters have different meanings, I am confused how one variable could mean so many different things, and where do I find the definition of that one variable.

Thanks!