The Publish Box on the add/edit post window, contains buttons and fields that control the state of your post.
The two primary states that you will need are Published and Draft. A Published status means the post has been published to the site for all the world to see. A Draft status means the post has not been published and remains a draft that only site administrators can view. If you select a specific publish status and click the Update Post or Publish button, that status is applied to the post. For example, to move a post to the Draft status, select Draft from the Publish Status drop-down box, and click Save Draft. You can view all posts organized by status by going to All Posts and selecting one of the links near the top of the page.
You can schedule a post for publication on a future time or date, by clicking “Edit” in the Publish Box next to the words “Publish immediately”. You can also change the publish date to a date in the past to back-date posts. Change the settings to the desired time and date. You must also hit the “Publish” button when you have completed the post to publish at the desired time and date. One reason to modify the date of a post is that posts often appear on pages in the order they were published. So to move a post to the top of a page, it is sometimes necessary to change its post date to be more current than all the other posts in the category.
See the WordPress Writing Posts documentation for more information on these features.