It was your use of the word "extend" that threw me off. 😊
http://www.theverge.com/2013/10/28/5036102/mac-os-x-10-9-mavericks-review
It’s rare for a nerdy feature like improved multiple-display support to be a major selling point for a new desktop operating system, but considering how much of a headache it’s been since Lion, there’s good reason to pay attention to the changes Apple has made here in Mavericks.
Using more than one display got complicated when Apple added virtual desktops (called "spaces") in Leopard, as Macs could no longer simply extend the usable desktop across two monitors like Windows does. Instead, each virtual desktop was extended across both displays, leaving you with multiple pairs of extended desktops. When Apple added long-overdue support for full-screen apps in Lion, things got much worse; opening a full-screen app on your laptop would leave your external display with a useless panel of gray linen.
Mercifully, the situation is a lot better with Mavericks. The desktops on your primary display and your external display are no longer bound to each other. You can see this when you enter Mission Control: if you have five virtual desktops on your laptop, your external monitor will then show up as "Desktop 6." And since they’re not paired together, you can switch between desktops on your primary display without changing anything on the external monitor. You also aren’t forced to have multiple virtual desktops on your external monitor anymore, and moving apps from your primary monitor to your external display is as simple as dragging it in Mission Control. (This was inexplicably impossible to do in earlier versions.) Full-screen apps now function properly as well: you can open an app in full-screen on any display, and the other monitor remains unchanged. (No more gray linen on entire displays.) And if you use two full-screen apps at the same time, it all works just as it should.
With Mavericks the menu bar is now on both displays, as it always should have been. The display in focus gets the standard menu bar, while the other monitor is left with a very transparent version. The menu bar you see corresponds with whatever app you’re using on each display; for example, it will give options for Chrome on your laptop while the menu bar on the external monitor will simultaneously offer choices for Finder. Apple’s also decided to duplicate the dock on the secondary monitor. It’s hidden, however, and you’ll have to hit the bottom of the screen with your cursor to make it appear.
Unfortunately, this rarely works, and I had to tap at least twice to make it appear. More importantly, I’m not convinced that there’s a good reason to have two docks.
If you just can’t get enough displays, Mavericks lets you use a television connected to an Apple TV or another AirPlay device as an external monitor rather than just a mirrored version of the primary display. It works impressively well, though you’ll need a newer Mac (from 2011 or later) to use the feature.