Different Screen Displays

[AC Question]

Hi, I have just purchased a Galaxy J5 phone and I'm new to the Operating System.
I am confused about some of the terminology - particularly that related to the different screens and how to configure them....
Can someone please explain - in simple terms what the various displays are, when they are presented and how to set them?
For example :
1. Desktop....
2. Wallpaper...
3. Lock Screen... (Fairly obvious!)
4. Backgrounds....
5. Themes....
6. Home Screen...

The Android Version is 5.1.1 (Lollipop)

Any help would be much appreciated.....