if ( !is_admin() ) { // instruction to only load if it is not the admin area
  // LOAD JQUERY   
  wp_enqueue_script('jquery');

  // LOAD EXTERNAL LIBRARY
  wp_enqueue_script('jquery-ext', 'http://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js');
}

http://codex.wordpress.org/Function_Reference/wp_enqueue_script

This code adds the specified library to the header in WordPress unless you are in the admin section.

Load JavaScript Libraries in WordPress

php, wordpress

PHP