Hi all,
Recently I have started exploring about linux kernel. I have few queries.
a) What is the role for initrd(ramdisk) and how it is related to the root file system?
b)When does the kernel actually link to the root file system and the desktop environment(such as gnome) is created?

Please let me know

Initrd is a initial ram disk for loading a temporary file system into memory in the boot process of the Linux kernel.

This process happen before mounting root filesystem.

For a beginner its very hard to understand but i try to simplify.

To mount the root filesystem the OS must understand the underlying hardware structure
IDE/SATA,SCSI ,SAN DISK.
filesystem architecture such as LVM, RAID

So if this is not understand properly in boot process ,the boot will fail.
Linux guys come up with idea of detecting all this by means of process which is nothing but initrd process .
In this process all the necessary drivers needed to complete the prerequisites to mount root filesystem is done.

Hope you understand.

As pinga mentions, the initrd mostly contains drivers that are necessary for the kernel to load the real on-disk root filesystem and pivot to it (the pivot to the real root happens after is the kernel is initialized, but before the init program is started).

The kernel actually doesn't do too much initialization beyong setting up the hardware (as if that isn't enough). Things like launching the GUI and starting services are generally handled by the init program, which is a special user space program with a PID of 1. It parses its configuration file (the details of which vary based on which version of init is in use) and starts services, login prompts (getty), and GUI shells as directed.

Thanks for the info