There's a keyboard shortcut to show the desktop on most (GNOME-based) Linux systems—Ctrl+Alt+D, or sometimes just Windows+D. If you like having an actual button to drop down, à la Windows, you can ...
If you come from the world of Windows, you undoubtedly understand the concept of a shortcut. In the Linux world, shortcuts do exist, but they're generally referred to as symbolic links, or symlinks.