Need advice on creating CCK-based nodes from external application

I'm developing a site where much of the content will be provided by an external application, rather than manually entered via forms on the site. Basically what I need is a kind of automated bulk import, i.e. no user intervention. Is there some kind of API for this kind of thing? Do I need to figure out how to write directly to the database tables?

My major concern is that the data involves several related custom content types with node-references and other custom fields, so I'm challenged with either hard-coding some of the data-type info into the external app, or figuring out how to acquire it from drupal/database each time. Any help (even just pointers in the right direction) would be greatly appreciated!