Initial Ram Disk
initrd is a temporary file system that's loaded from boot into memory. It holds the main init program that the kernel runs.
- It starts the init process which starts the systemd.
- systemd then takes over the next process.
initrd from Operating System
initrd is Operating System specific. Each distribution builds its own initrd during installation.
The kernel only expects an initrd to exist with a /init file inside. It then just runs that file.
- Kernel - is common
- initramfs - distribution specific