Hello ,

I had CF card ..on which I had Linux Debian 4.0 which runs as
Linux kernel in RAM(ofcourse) with rootfilesystem on RAM ..one ext3 partation on CF card with contains kernel image and rootfilesystem image.other partation is also ext3 type.with home and usr directory...

I mount first partation as readonly ...and other read write..

I would like to know what is difference between
.....
Mounting a filesystem (ext3,ext2) read-write filesystem as Read only
And mounting a readonly type filesystem (cromfs,cramfs) ....

I want to protect my CF card..

There won't be any write commands for the mounted filesystem itself. One may be able to write to the raw device however, e.g. /dev/mmcblk0.
The mount command won't accept the the rw mount option if you are mounting it with a read only filesystem.