Android is built in a way that allows “core” portions of the operating system to be replaced with applications written by a third party. That is, someone not the manufacturer, or Google itself. Nowhere is this more evident than the various Launcher (or Home) apps you’ll find in Google Play. The user-facing portion of the operating system — the home pages, the widgets, the application drawer and the icons — can be replaced in one fell swoop.
This is a good thing because chances are once you get a few Android fans together, you’ll likely see a big difference in the way they want their phones and tablets to look, act and feel. Inevitably, Nova Launcher gets mentioned.