Users coming from MS Windows might be surprised that in OS X, you can work with background windows without bring them to the front.
One obvious example is the ability to scroll the web page content in Safari window in the background, while you compose a message in Mail app in the foreground. This allows you to reference content in another app without having to switch window.
With the Command key, you have greater control working with background window other than scrolling window content.
You can move a background window by Command click and drag on the title bar. Command click and drag on corner and edge of background window will allow you to resize them. You can also close a background window by Command click on the close button.
When Safari window is in the background, Command click on a link on the web page will open the link in a new tab.
Command + right-click on a background window will popup the contextual menu the background window supports. Lets see how we can use this to perform a basic copy and paste of content in background window.
With Safari in the background and a word processor TextEdit window open in the foreground, hold down the Command key and click and drag to select the text in Safari. Command + right-click on the selected text to bring up the contextual menu, select Copy from the menu. The text you want is now on the clipboard ready for you to paste into TextEdit.