Rasbian is the officially recommended Linux distributions for Raspberry Pi. raspbian stretch is based on the Linux distribution Debian, which puts a lot of emphasis on stable and secure operation. At the same time, a lot of software is already delivered pre-installed. In addition, one has access to a repository (software archive) and can install many programs without difficulty.

Note: Debian is a Linux distribution created by Ian Murdock based on the Linux kernel and GNU tools. Debian is popular with many administrators because of its stability-oriented release policy. On the desktop, the variant Ubuntu is relatively widespread.
Official Download Source: Download Raspbian