Why some objects are not rendered in the final rendering? So this is how it looks when I get a preview rendering in the 3D viewport:   https://imgur.com/a/7r0Wk1k

This is how it looks when I press render for a final result: 

https://imgur.com/a/rzbA0eO

I have no idea what's happening. Can someone help me figure the problem out?
I figured it out. I have to go to the OBJECT MODE -> W -> Clear

Objects can be set to 'Restrict Render' - effectively to exclude them from the render (the camera icon in the Outliner window). The W shortcut brings up a Specials menu with 'Restrict Render Unselected' and 'Clear All Restrict Render' options - the first option sets all objects except those currently selected to be hidden, while the second option sets everything back to render.