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Unity is the most popular desktop amongst users of Ubuntu, developed by Canonical, Ltd. It aspires to converge phone, tablet and desktop into a single platform with both a productive desktop and a lightweight phone. Unity uses a global menu, akin to OSX, and a launcher bar, which defaults to the left-side of the screen, with the added benefit of a fuzzy search for menu items. Pressing the 'super' key also provides access to a more full launcher window, including search for files and applications.
SpecsUpdate
Pros
Pro Best keyboard bindings on desktop
Unity won a recent assessment of productivity for desktop users with the richest collection of keyboard shortcuts. Programs can be easily accessed by clicking on the Ubuntu symbol on the upper part of the launcher, or by pushing the "Windows" button found on most PCs. This opens up a box called the "dash" where users can search by typing in the name of desired programs to open them.
Cons
Con Dead
Canonical has ceased the development of the Unity desktop environment, along with the Mir display server. Future versions of Ubuntu will be shipping with Gnome 3. While branches of these are likely going to be maintained by the community, it's difficult to say how strongly these will last considering they were designed specifically for Ubuntu, they had never reached stable release, and more popular alternatives are already present (Wayland in the case of Mir).
Con Rubbish UX
The start button is right next to the windows button, so when you want to click the unity start button, you often end up closing an application, or clicking the browser back button because those buttons are just pixels away from the start button. Therefore, the UX/UI is very very poorly designed.
Con Files Manager lacks many features
The files manager based on gnome nautilus lacks too many features! With the removal of features such as compact view and split screen, newer versions of Nautilus File Manager are not as useful as they once were.
Con Performance is bad, even when running on beefy hardware
It starts fast, but quickly slows down once you use the system heavily. Using faster computers doesn't help for some reason. This is running on a laptop designed for Ubuntu (System76 Oryx Pro) with i7 quad core, 32 gb ram, gtx 980m 8gb vram, ssd drive. So it's not a matter of weak or incompatible hardware or misconfigured installation. On this machine, sometimes it takes a couple of seconds to do a window spread animation (Super+w). Doing Alt+Tab is sometimes not as responsive as it should be.