More information on initramfs can be found at Linux Kernel Documentation. If rootfs does not contain an init program after the embedded cpio archive is extracted into it, the kernel will fall through and try to locate and mount a root partition on its own, and then exec some variant of /sbin/init out of that (Rob Landley, 2005).
If found, this init process is responsible for bringing the system the rest of the way up, including locating and mounting the real root device (if any). After extracting, the kernel checks to see if rootfs contains a file 'init', and if so it executes it as PID 1. Now, Linux kernels contain a compressed “cpio” archive, which is extracted into memory file system (typically tmpfs) and used as a root filesystem. Introduced in Linux 2.6 series, initramfs is the successor of initrd. Initial RAM file system, abbreviated as initramfs, allows the kernel to run user space applications from a RAM disk built-in into the kernel or passed by the boot loader.
