Raspbian is a Linux Debian variant for Raspberry Pi.
All release notes for Raspbian are here:
Raspbian Stretch Lite is a minimal Raspbian variant based on Debian Stretch. This contains the packages you need to run the operating system in the lightest possible way. This is Raspbian without a graphical user interface, desktop.
Raspbian needs to be updated after booting.
Then enter in terminal:
sudo apt-get update
sudo apt-get upgrade