"The name initrd is an abbreviation for “Initial
RAM disk”. It’s a file, which includes the image of a
file system. The idea is this: RAM disk (that is, a carefully
selected chunk of memory which is visible for everyone else as just
another data device - it could be described as a virtual hard
drive) is kept in the computer memory. Its content is kept in the
initrd file. Now the obvious question comes: why do all that? Why
can’t we simply use the hard drive directly? Well we could,
but the virtual drive has its indisputable
benefits..."