It seems that Google has taken a page out of Paranoid Android's book here and was perhaps intending to release a new feature at some point called "head's up", that is basically a floating notification window that can pop up over the existing active app or area on your phone. For those of us that may already be using Paranoid Android and have been for the past few releases, this might feel quite a bit similar to the Hover feature that the PA team has been implementing in the latest Kit Kat builds of their ROM since earlier this year. This Heads Up feature looks and even seems to function very similarly to Hover, in that a notification can be displayed in a top layer panel over whatever else is on screen at current.
While Hover will display a brief few seconds of the notification up at the top of your screen when it comes in, and expanding only if you proceed to tap on that Hover bar, the Heads Up panels that we see pop here in these images actually seems to show up whenever the notifications come in, so the idea may have been to allow for notification panels to just appear as they come in, instead of tapping on the notification up top to display them like with Hover. This new Heads Up feature was apparently laying dormant in the AOSP code and was discovered by the team at CyanogenMod, which they have now implemented into the latest build of CM nightlies, which is where these images are actually from.
With Heads Up, the feature can be disabled entirely if you don't want to be bothered by notifications popping up on top of other apps, or you can simply poke around the "do not disturb" feature that lets you basically blacklist certain applications that you don't want to pop up, while letting others through. So if you don't mind your email notifications popping up you can leave those be but ask your SMS to not disturb you via the feature, and you can continue to interact with those notifications the same way you would normally. For those that are interested in how Heads Up works, it's basically just the notification list ported to your homescreen. You can interact with them in the same way you would if they were still in the notification pulldown, and you can swipe them away to dismiss them and touch and drag to extend them for more information or to interact with them further. Now that CM has found this last week, we have to wonder if it will be making its way into stock Android at some point.