|*buntu uses busybox in its initramfs. Initramfs is a very, very small Linux system that mounts certain drivers and other utilities--basically makes sure the hardware is turned on and functional--before booting the rest of the system after switching to the main part of the operating system. Busybox provides the necessary tools for this process without the bloat of using coreutils or comparable, full-size packages.
I've really oversimplified this. If y'all want a more technical explanation, I'll post again with my best understanding of it.
Sorry it's taken me a while to get around to this explanation.
You can think of it like this:
The bootloader (GRUB or LILO) loads the kernel into RAM. In a way, the bootloader tells the kernel to load a file that contains a very, very basic Linux filesystem. This filesystem contains busybox, uclibc, and a few other things. It's more used to run fsck and other utilities on the hard drives before they're mounted.