To create a new template, simply copy/paste the default one, and override the HTML to reflect your modifications.

Here is a list of files and their purpose:

codex_crumbs.php

Loads the breadcrumbs in the header.

codex_delete_confirm.php

Loads a delete confirmation page. This has been deprecated, but is there if your clients request it.

codex_form_element.php

The template for plugins.

codex_form_view.php

Loads the form for addition, and updates.

codex_header.php

Pulls together the header, navigation, crumbs, and search form.

codex_login_view.php

The login form.

codex_permissions_form.php

The permissions form that appears at the bottom of the Add New and Edit forms.