I assume you are using KDE from the tag. First, some background: KDE4 doesn't have a "desktop" per se, but uses a "widget dashboard". Users can make this behave like a traditional desktop, by using "folder view". There are two similar ways of showing this dashboard/desktop: either as a transparent overlay over all of your windows ("dashboard"), or by hiding all the current windows on screen ("desktop"). Each can be triggered by a keyboard shortcut, by a mouse-click on a widget (plasmoid), or by pushing the mouse cursor into the corner of the screen.
To show the widget dashboard, the default keyboard shortcut is Ctrl-F12. This can be changed in System Settings > Shortcuts and Gestures > Global Keyboard Shortcuts > KDE component = Plasma Desktop Shell > Show Dashboard.
To hide all the windows, customise the keyboard shortcut in System Settings > Shortcuts and Gestures > Global Keyboard Shortcuts > KDE component = KWin > Show Desktop.
I don't know how it works in Windows 7, but you enable a widget in the panel. To show the widget dashboard, use the "Show Widget Dashboard" plasmoid. To do this, click on the cashew on your panel, then "Add Widgets". Search for "dashboard", then drag the icon to an appropriate position in your panel.
To hide all windows, use the "Show Desktop" plasmoid. Note that searching for "desktop" doesn't seem to find it, so just scroll across until you can see it.
Screen corners (or edges)
System Settings > Workspace Behaviour > Screen Edges > Active Screen Edge Actions. Click on the icons at the corners/edges of the screen picture, and select Show Dashboard/Desktop.