In the beginning, there were iOS, Android, and the Web. Entirely separate platforms that had to be developed as such.
Then, along came Xamarin. Developers could write iOS and Android apps using a single codebase, but we were still on our own for Web development.
Now, Uno has emerged. Building on top of Xamarin, it gives us the power to write iOS, Android, Web, and even UWP applications using shared logic and UI!