Windows XP Desktop

After logging on to Windows XP, Vista, 7 or 10, Desktop appears. Basically, it looks the same in all versions of Windows from XP to 8.1.

Windows 8 and 8.1 display the Modern UI/Metro-style Start screen after signing in. Because it fills the whole screen, it is called Start screen, not Start menu. Windows 8 Desktop omits Start button, but moving mouse pointer to the left bottom of screen still offers to open the new Start screen. Windows 8.1 brings the button (called "Start tip") back, but this is just a shortcut to Start screen. You can still get to Desktop quickly in Windows 8 and 8.1 using the WINDOWS KEY+D shortcut.

Let's take the good old Windows XP Desktop as an example:

Desktop area is where your Desktop icons and open programs appear. Here you can open programs and folders by double-clicking on their icons. To minimize all program/app windows and display Desktop, press WINDOWS KEY+D on your keyboard.

By default, Desktop shows only Recycle Bin icon. You can drag any program icon from Start menu there. To add shortcuts to files or folders, right-click an empty area of Desktop, choose New and Shortcut.

Desktop background/wallpaper and Windows theme can be changed by right-clicking or touching and holding an empty area and choosing Properties or Personalize.

Start button opens Start menu with pinned items, recently added and/or most used programs, Search box (in Windows Vista, 7 and 10), All Programs or All apps list and shortcuts to common folders and places. You can press WINDOWS KEY to open Start menu (or Start screen), or you can use the CTRL+ESC shortcut if there is no Windows Key on some keyboard.

To pin a program or an app to Start menu or screen, right-click or tap and hold its icon and choose Pin to start (menu).

To remove an item from the most used programs list, right-click it and choose Remove from This List (Windows XP, Vista and 7), or More, Don't show on this list (Windows 10).

In Windows 8, there is no Start button visible on Taskbar, but if you move mouse pointer to the bottom left, the colorful Start screen button appears. Clicking it opens Start screen, right-clicking it opens Quick Links menu with access to system management tools. The latter is also available in Windows 8/8.1 and 10 using keyboard shortcut WINDOWS KEY+X.
 

Windows 8.1 adds Start "tip" with Windows logo back to the leftmost corner of Taskbar, but it will still open Start screen, not classic Start menu. The "tip" will also be displayed outside of Desktop area (for example, while an app is running) anytime you move mouse pointer to the lower left edge of screen.
By default, Quick Links menu in Windows 8.1 and in Windows 10 Creators Update (April, 2017 - version 1703) has Windows PowerShell shortcuts instead of Command Prompt ones. This can be reverted in Taskbar's Navigation settings (Windows 8.1) or Taskbar settings (Windows 10 Creators Update), but all Command Prompt commands do run in PowerShell, too.
Please note that Shutdown/Power options are back in the menu in Windows 8.1. Yay!
Windows 10 Creators Update replaces the Programs and Features item with newer, Modern UI based Apps and Features entry for managing installed Windows Store apps and desktop programs. Additionally, Control Panel has been replaced by the Modern UI Settings app.
  

Windows XP, Vista and 7 Start menu can be personalized by right-clicking the Start button and choosing Properties.
Windows 8 and 8.1 Start screen can be customized only while it is open. Either move your finger or mouse pointer to the lower right side of screen and choose Settings; or use keyboard shortcut WINDOWS KEY+I.
In Windows 10, open the Settings app and navigate to Personalization, Start.

Quick Launch Bar / Quick Launch Toolbar includes buttons to launch favorite programs. You can rearrange the buttons using your mouse - just click and hold on a program button and drag it to a different position.
In Windows Vista, 7, 8/8.1 and 10, you can launch the first five programs or apps here using the WINDOWS KEY+<number>shortcut. For example, if Windows Media Player is the third button from the left, open it with WINDOWS KEY+3.

To add programs and apps to Quick Launch, either drag its icon from Desktop or Start menu there, or right-click/tap and hold the icon and choose Pin to taskbar. The latter is not available in Windows XP only.

Taskbar contains buttons/icons of running (currently open) programs/apps. You can bring these to front or minimize, maximize and close their windows here. To cycle through all open programs and applications, use keyboard shortcut ALT+TAB.
Windows Vista and 7 allow using special 3D effects with keyboard shortcut WINDOWS KEY+TAB.
In Windows 8 and 8.1, the shortcut cycles through open Windows Store/Modern UI/Metro-style apps only.
In Windows 10, this opens Task View that allows switching between apps and different virtual desktops.

Windows 8 and 8.1 list only running Desktop programs on Taskbar (and Taskbar is not available in Modern apps); Windows 8.1 Update and 10 also display open Windows Store (Modern UI/Metro) applications, plus makes it possible to reveal Taskbar in Modern apps by moving mouse pointer to the bottom of screen, or by using keyboard shortcut WINDOWS KEY+T.
Windows Vista and newer allow cycling through Taskbar applications/icons using the WINDOWS KEY+T shortcut.

Right-clicking or touching and holding on an empty space of Taskbar and choosing Properties (Windows XP, Vista, 7, 8/8.1 and the original Windows 10) or Settings (Windows 10 Anniversary Update) allows customizing Taskbar to your likings.

Notification area (aka System Tray) includes icons of some running programs and important messages. It also includes current date (in Windows XP) or current time and date (in Windows Vista, 7 and 8/8.1). To customize which program icons are always visible, right-click or tap and hold date/time in the lower right part of screen and choose Customize Notifications (Windows XP) or Customize notification icons.

Windows Vista and 7 also have Sidebar that holds Desktop Gadgets. You can use keyboard shortcut WINDOWS KEY+Gto bring Gadgets to front temporarily. Windows 8 and later deprecate Desktop Gadgets due to numerous security concerns and replace them with Modern UI/Metro-style apps.
Here's a Windows Vista example of Sidebar:

As usual, Sidebar and Gadgets can be managed with right-click menus.

Windows 7, 8, 8.1 and 10 have a convenient button on the very right of Taskbar's Notification Area - Show Desktop. Stopping mouse pointer on it makes all windows transparent and you can see Desktop icons and frames of open windows (this is called Peek at Desktop). Clicking the button minimizes all open windows and displays Desktop. Clicking it again restores open windows.
The latter also be achieved with WINDOWS KEY+M and WINDOWS KEY+SHIFT+M keyboard shortcuts.