Initramfs

An initramfs (initial ram file system) is used by Linux systems to prepare the system during boot before the operating systems' init process starts.

The initramfs usually takes care of mounting important file systems (by loading the proper kernel modules and drivers) such as or, preparing the  file structure, etc. Users who use an encrypted file system will also have the initramfs ask them for the passphrase before it can mount the file systems. When the file systems are mounted, control is passed on to init which then takes care of further starting all necessary services and booting up the remainder of the system.

Building an initramfs
Several ways :
 * Early Userspace Mounting
 * Dracut
 * Genkernel
 * Mkinitcpio
 * mkinitramfs-ll, sys-kernel/mkinitramfs-ll
 * Custom Initramfs

Resources

 * Gentoo Linux Initramfs Guide