What is the difference between the Android Official rom stock and unofficial Stock rom?

Question

Example, Kit kat, lolipop, jellybean or any version. They may be official or unofficial, but is there any actual difference between them?

Answer 1

You often hear the term Stock OS. Means it is pure android, without any customisation. Like in some Motorola devices, you find this Stock OS and you can call MiUi a custom android OS.