Our MacBook came configured with
OS X 10.10.3, the latest version of Yosemite. It's the eleventh major release of OS X, and the second version since switching the naming scheme from jungle cats to landmarks in California.
For this overhaul, the most noticeable change is the flattened design compared to Mavericks, much like Apple did with iOS 7. Apple also introduced several visual tweaks such as semi-translucent windows that adjust their color temperature based on the background. For example, if you open up Finder, the window is translucent enough to let the background shine through, but the background is blurred so that you can focus on the icons. This is something you can adjust, by the way.
The Notifications Center now has a Today view that summarizes the day's events, offers reminders, and shows tomorrow's schedule. You can also customize the view with widgets. This is yet another nod to iOS, which also has a Today view and supports widgets.
With Yosemite, Apple is making a big push towards what it calls Continuity. This simply means the ability to perform the same tasks on multiple Apple devices, while also being able to hop from one to another and pick up where you left off. Maybe you're surfing the web or hammering out an email on your MacBook, but it's almost out of battery life and you're not close to an outlet. You could whip our your iPhone or iPad and finish that email or surf the web from wherever you left off.
This interactivity between devices is perhaps the best part of Apple's ecosystem, provided you're going all-in with Apple. That doesn't mean OS X Yosemite isn't great if you're toting an Android phone or tablet, you just won't be able to take advantage of the full feature-set.