To save our bandwidth, we show only a snippet of code around each occurence of the hook. View complete file in SVN (without highlighting).
The best way to understand what a hook does is to look at where it occurs in the source code.
do_action( "hook_name" )
apply_filters( "hook_name", "what_to_filter" ).
Remember, this hook may occur in more than one file. Moreover, the hook's context may change from version to version.