Wordpress - Can posts have parents?

Out of the box, no, posts can't have parents. They can be assigned to categories, which can be organised hierarchically. Pages, however, can have parents and you can build a menu structure out of them by using that feature.

As to where this is enforced: The parent of a post is stored in the column "post_parent" in "wp_posts". It's not really enforced as such, just the default UI doesn't give you an option to set it and default WP coding doesn't use that value for Posts. It wouldn't be too difficult to create a custom post type to have posts with parents though.

Wp has built in "Pages" (hierarchical, parents allowed) and "Posts" (non-hierarchical). There are also other post types, but let's leave that away.

If the Q results in:

Can I have hierarchical posts?

Then the answer is Yes,… you can have "posts" that are hierarchical. But as they are not built in, you'll have to register your own Custom Post Type - see Arguments » hierarchical.

Such "Posts" (or articles, whatever, …) will then - in case they have a parent post - have set the parent ID inside their object. So in a loop you could do the following:

if ( have_posts )
{
    the_post();
    // etc.

    global $post;
    // call parent: http://codex.wordpress.org/Function_Reference/get_post
    $parent = get_post( $post->post_parent );

    echo "<h2>{$post->post_title} is a child of {$parent->post_title}";

    // etc.
} // endif;