Start Screen and Start Menu change how your desktop looks in Windows 10. Some people like one way and some like the other way. Lets first look at what each is and then how to change
Start Menu gives you the desktop like it looked in Windows 7 and before (of course it has change some along way, but same basic look). With this set up when you click the Start button (the button in lower left of screen normally) you get a menu listing the programs on your computer in a format using words. This menu looks like what you had in Windows 7.
Start Screen gives you a desktop that looks like Windows 8 and on your phone and usually on tablets. This has large tiles or icons across the screen which you click to start your various programs. (In the Start Menu until a recent update of Windows 10 this was presented to the right of the Start Menu also). In this setup you do not have the small icons all over the desktop for the various programs and shortcuts like Start Menu does, but only the large tile which can have smaller pieces in them. This approach is also called Tablet Mode by Microsoft. If you are working using a touch screen (as on a tablet or phone) this one can be easier as larger objects.
You can easily switch between Start Menu and Start Screen (if you are not sure what other is you can try both). There are two ways to do this switch.
The first way is open Notifications (button on far right of Taskbar (which is bar across bottom that has the date on it) and you will have a window open on right of screen which will have any notifications Windows has sent you. On the bottom is a button that says Tablet Mode. If it is grayed out, you are in Start Menu Mode. If it is blue you are in Start Screen mode. You may not be able to change it if you do not have a touch screen.
The second way is to go to settings and change it. Press the Start button, click the link that looks like a gear (settings), choose Personalization, click Start in the left part of the window, and click Use Start Full Screen to turn on the switch. Now close the windows and you are in Start Screen. Now when you click start you get the full screen of icons if you have a touch screen it will now be your desktop.
To go back to Start Menu just do those directions and turn off instead of on.