Hi Peerj,

Please find my answers from your inquiries below.

Buttons Apply and Publish Post and Update Post
What these three buttons have different functions?

Apply button is used to publish your blog post but it will not redirect the user to anywhere after that. If you click on publish post, you will be redirected to dashboard page after that. Update button is used to save and publish your current editing blog post and then redirected you to the dashboard page. Hope these can clear up the confusion.

Can i decide whether to Publish My Post?
Because each of these three buttons will automatically Publish my Post.

You can click on "Save For later" button instead to save your blog post as draft post and this post will not get published until you decide to do so. http://screencast.com/t/CxAIkQD2Lb0R