This is a simple module that adds a "node-content" theme hook to Drupal for theming the $node->content array. For example, with this module enabled, you can create a "node-content--page.tpl.php" file in your theme's folder, giving you templating control of the "Page" content type. This can be used for any content type, but the use-case I expect it to be most useful for is in having greater theming control of content types that use CCK fields.
There's another module, contemplate, that solves a similar use-case. That module, however, is more geared to providing template control via the CMS interface, whereas this module is more geared to providing it via the normal theming system. Pick the one appropriate to the way you like to work, or mix and match.
Instead of this module, you can take the approach of using "node-<type>.tpl.php" files, as described on http://drupal.org/node/62462. However, then you end up duplicating all of the generic markup that deals with the node title, links, terms, and publish status, that you don't necessarily need to have customized per type. If you want to customize all that generic stuff per type, use those tpl files. But if you only need to customize the node's type-specific content, then this module might be useful for you.