How to promote nodes/pages into blocks? Standard Drupal comes with a "promote to front page" when creating new content. But I want know how to promote, let's say News flow to the block located to the right?

As far as I know there is no "url" for blocks so I can't really use view to assign my news flow to the blocks url. So how do I do this the easiest way?
I complete the previous answer about Drupal 8.

In Drupal 8 with Views (include in core) it's exactly the same way.

Add a 'Promote to block' checkbox field on your News content type.

Then in Views you can create a new View or you can go in the "Content" View. Then create a new "Block". Describing all Views functionalities would take a long time, but in this block you can filter all your content, particularly in the Filter Criteria sections where you can filter in order to take only News content type with only the 'Promote to block' option checked.

If needed, search for some Views tutorials. It's quite easy and very powerful.

Then call this new block instead the old one.