You can create password protected pages and you can link to them in your custom emnu. Doing so will result in a visible tab in your horizontal navigation but only those with the password will be able to access the content on the page.
Protected – The page is protected with a password you set. Only people who have the password can view a protected page.
It will still be a page with the characteristics of a static pages. It will not be a a specific part of the blog. You will only be able to update it manually.
http://en.support.wordpress.com/pages/
The differences between pages and posts
http://en.support.wordpress.com/post-vs-page/
Staff have provided this support documentation entry for those who want to structure their blog like a website. > http://en.support.wordpress.com/using-wordpress-to-create-a-website/ We organize our posts by use of categories. There is only one dynamic page in a blog for posts and we cannot post to more than that one page. However, we can create the appearance that we have done that by setting up a custom menu and displaying Categories in tabs along the horizontal navigation menu. Instructions > custom menus > http://en.support.wordpress.com/menus/
Read also> http://wpbtips.wordpress.com/2010/06/30/posts-on-pages/