Custom Queries

This query is taken from the codex on how to make custom queries. Can someone explain to me what purpose the wpost and wpostmeta serves? There are no tables with those names in the database.

$querystr = "
    SELECT wposts.*
    FROM $wpdb->posts wposts, $wpdb->postmeta wpostmeta
    WHERE wposts.ID = wpostmeta.post_id
    AND wpostmeta.meta_key = 'custom-key'
    AND wposts.post_type = 'page'
    ORDER BY wpostmeta.meta_value DESC
    ";

Thanks