If you want to display Login form on your website then you can display that by just single line of code.

WordPress added new function using which you can easily add wordpress login form anywhere on your wordpress blog.

Function

<?php wp_login_form(); ?>

Usage

 <?php $args = array(
        'echo' => true,
        'redirect' => site_url( $_SERVER['REQUEST_URI'] ), 
        'form_id' => 'loginform',
        'label_username' => __( 'Username' ),
        'label_password' => __( 'Password' ),
        'label_remember' => __( 'Remember Me' ),
        'label_log_in' => __( 'Log In' ),
        'id_username' => 'user_login',
        'id_password' => 'user_pass',
        'id_remember' => 'rememberme',
        'id_submit' => 'wp-submit',
        'remember' => true,
        'value_username' => NULL,
        'value_remember' => false ); 
wp_login_form($args);
?>

More Details

For more details read WordPress Codex.