Difference between Arduino Release and Arduino-git?

Just an entry-level question:

There are definitely differences between Arduino Software Release (currently, it's 1.6.10, https://www.arduino.cc/en/Main/Software), and Arduino-git (GitHub - arduino/Arduino: open-source electronics platform)

But, clearly, for Arduino-git, there is NO such a file ./install.sh
Anyway, we can still run Arduino within Arduino-git by command ant run, referring to

Anyway, can anybody help to explain the difference between Arduino Release and Arduino-git please? For most of the other software packages, we can either install it directly, or git clone the source code out from github, and have it built and installed manually by ourselves. Any suggestions?

Thank you very much.

Pei