Initial Ram Disk
initrd is a temporary file system that's loaded from boot into memory. It contains the main init program that will be executed the kernel
- 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.
Kernel only expects an initrd to be present and a file /init to exist inside it. Kernel then simply executes that file.
- Kernel - is common
- initramfs - distribution specific