The booting in Debian is a two-stage process, involving the initial RAM filesystem (initramfs for short, sometimes it is also referred to as initrd, which stands for initial RAM disk). First, the bootloader loads the kernel and initramfs into memory, and passes the execution control to the kernel. After basic initialization the kernel extracts the initramfs archive and mounts it as a temporary root filesystem. initramfs contains kernel modules and userspace programs required to initialize the physical or logical device(s) containing the real root filesystem. The init script on the initramfs loads modules and performs other neccessary initialization steps. At the end of this stage run-init deletes the initramfs from memory, mounts the real root filesystem and passes control to the /sbin/init program on it.
Two major goals are achieved with such setup: the kernel size is kept under control by allowing most of the drivers to be compiled as modules (in a initramfs-less setup the drivers neccessary for the boot-time initialization of the root device must be compiled into it) and allow the setups which require initialization which cannot be done in-kernel, but is performed by userspace utilities.
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