OS X is great at running lots apps at once, many of which can interact with one another. Of course we have Exposé to handle switching between windows, but what happens if you don't want to switch to another window, but rather maintain the focus of your current window while doing something in another. Well, by now you've probably guessed that if I'm writing about such a scenario, it is indeed possible. Read on to see what you can do in "background windows" and how to go about doing those cool things.
The first thing to know is the how. As Jeff Gamet pointed out in a tip several years ago, you can easily scroll windows in the background by simply hovering your cursor over them and using the scroll gesture on your trackpad or mouse. To do any of the other tricks with background windows (and keep them in the background), you'll need to hold the Command key. In it's most basic form, this will allow you to move windows around in the background without bringing them to the foreground. This is nice if a close button you're after is hidden behind something else and you'd like to move it to a more accessible spot.
But you can do more. You can actually do many things in a background window that you normally do with foreground windows, like press buttons, click links, check boxes, and open folders just by Command-clicking. However, because some actions actually utilize a Command-click, you may not get the expected result. For example, clicking a link in a backgrounded Safari window will open the link in a new tab just as it would otherwise.
As for the practical uses for this, your mileage may vary. I primarily use it for gathering notes from websites for my podcast. My primary window is a TextEdit document, into which I drag links from a Safari window populated with tabs full of interesting articles for discussion. Other than that operation, I use it sporadically. However, the regular use of the feature in this way has sharpened my muscle memory for its use in other, more random cases.
Go ahead and give the feature a try, it may become extremely useful. Enjoy!