The best way to understand what a hook does is to look at where it occurs in the source code. 

1. Action hooks look like this: do_action( "hook_name" )
2. Filter hooks look like this: 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. (Source/more: Adam R. Brown)