add_action();
remove_action();

//* Generic action
add_action( 'hook', 'callback_function', 10, 1 );

//* Generic callback function
function callback_function( $arg1 ) {

   // Let's do some stuff!

}

//* Generic actions with different priorities
add_action( 'hook', 'function_a', 10, 1 ); //executed 2nd
add_action( 'hook', 'function_b',  5, 1 ); //executed 1st
add_action( 'hook', 'function_c', 15, 1 ); //executed 3rd

//* Generic actions with same priorities
add_action( 'hook', 'function_a', 10, 1 ); //executed 1st
add_action( 'hook', 'function_b', 10, 1 ); //executed 2nd
add_action( 'hook', 'function_c', 10, 1 ); //executed 3rd