Every category has a Show on frontpage setting. Turn it off and the category becomes a quiet corner of the site: its posts stay out of the front page and the main RSS feed, but they're still here.
Its posts can still be found:
- on this category's own page, which stays in the menu
- in this category's RSS feed
- in search results
- on the author's page
It's useful for site news, niche topics, or anything you want to publish without filling up the front page. A post can also be in several categories: if any of them is shown on the front page, the post is too.