The lock screen is the screen you first see when your Windows 10 computer or device has booted up. Click it, and it displays the form into which you enter your password to log in.
Example of Lock screen
The desktop (also called start screen on phones and tablets) is the everyday workspace for your computer - the first screen that shows up after you've logged in. This is the screen that contains the taskbar, any icons you wish displayed, along with personalization such as a background or wallpaper.
Example of Desktop
Both screens can be customized as to:
Customization of both screens is discussed in detail in Settings > Personalization, under Lock screen and Background.
In addition, the desktop and Start Menu are highly customizable as to what they can do and how much can be made available directly from those screen locations. You can review a detailed discussion about this on the Desktop and Start Menu (Details) page on this website.
Dynamic Lock is a feature based on linking your computer to your cell phone (Bluetooth capability on both is required). It also assumes you carry your cell phone with you at all times. When Dynamic Lock is active, your lock screen will be automatically activated if you (and your cell phone) move more than about 25 feet from your computer, thereby securing your computer. See the Dynamic Lock page on this site for details.
Windows 10 allows for Virtual Desktops: you can "clone" your desktop and create a second, third, or even more desktops, each of which can be customized for a specific purpose. Examples would be: one for general use, one to help you pursue a hobby, one for household financial work. There is additional discussion of Virtual Desktops, including how to set them up and a discussion of their limitations (e.g., their content does not persist), on the Virtual Desktop page of this guide.