i am new to linux. why my grub.conf shows uuid id /dev/sda3 as boot partition is mounted on /dev/sda1
# grub.conf generated by anaconda
# Note that you do not have to rerun grub after making changes to this file
# NOTICE: You have a /boot partition. This means that
# all kernel and initrd paths are relative to /boot/, eg.
# root (hd0,0)
# kernel /vmlinuz-version ro root=/dev/sda3
# initrd /initrd-[generic-]version.img
title Red Hat Enterprise Linux (2.6.32-358.el6.x86_64)
kernel /vmlinuz-2.6.32-358.el6.x86_64 ro root=UUID=6ed86489-d641-40b9-9b 8d-c9d653eb731f rd_NO_LUKS rd_NO_LVM LANG=en_US.UTF-8 rd_NO_MD SYSFONT=latarcyrh eb-sun16 crashkernel=auto KEYBOARDTYPE=pc KEYTABLE=us rd_NO_DM rhgb quiet
[root@localhost ~]# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/sda3 16G 2.4G 13G 16% /
tmpfs 932M 80K 931M 1% /dev/shm
/dev/sda1 291M 37M 240M 14% /boot
[root@localhost ~]# blkid
/dev/sda3: UUID="6ed86489-d641-40b9-9b8d-c9d653eb731f" TYPE="ext4"
/dev/sda1: UUID="1328fa5e-594e-4ca9-8e0c-f2f2898d6e33" TYPE="ext4"
/dev/sda2: UUID="9ea22237-e2ef-406c-aca5-b6a8ddc19d98" TYPE="swap"
Answer to the question
Atanere is correct except he swapped sda1 and sda3.
You have a single drive. It appears to have had Windows loaded originally and that is why you have a very small sda1 partition, windows always creates these ( but that's another story) ... It is using this partition to house the boot information, including your active boot record and menu for grub (if you have one)... the main "chunk" of your sda3 and is where root filesystem "/" is and everything else is created or mounted in reference to here. Sda2 is a separate partition and is where the swap space is mounted. This is normal.
Answer to the question
I don't think you're interpreting the information correctly, and I'm not sure that I can either. I think it would help to know more about your system... desktop? Laptop? What kind of hard drive? Does it also have Windows or some other OS?
The drive sizes reported by df -h seem extremely small. Without knowing more, I'd have to guess you have some kind of netbook with 16GB "flash storage" instead of a regular hard drive or SSD. So /dev/sda1 is the largest and is mounted a / -- thats where everything is, including /boot where vmlinuz is stored.
And my guess about /dev/sda3, at only 291 MB, is that it is the UEFI boot partition. It may actually be at /boot/efi -- but I'm not sure. Newer computers have to get past the UEFI hurdles before they can actually boot the Linux kernel, or any other OS for that matter.
Sorry to not be more helpful, but good luck!