Before reading the API, you should refer to http://drupal.org/node/119576, which offers a short tutorial on MySite plugins.
Please be sure to also read http://drupal.org/node/318 for Drupal coding standards and refer to http://api.drupal.org for core Drupal documentation.
The best starting point for this API is probably http://therickards.com/api/group/hooks/MySite. This section shows you the common functions used by MySite plugins.
If you find errors in the documentation, please file an issue at http://drupal.org/project/issues/mysite.
-- agentrickard