Each storage device has an MBR/MPR area. It is the first 512 byte sector on the drive or storage device. Actually, the first 446 bytes is where the grub kernel or other boot pointer/routine goes. The last 66 bytes of the first 512 byte sector is the MPR, (Master Partition Record). Each storage device, real or virtual, is numbered in grub notation starting at 0. Example: (hd0) = sda, (hd1) = sdb, (hd2) = sdc, etc.
Partitions on a storage device are numbered in grub beginning with 0, and are the second number in grub notation. Each partition has its' own area where the grub kernel or other boot pointer/routine goes. Example: (hd0,0) = sda1, (hd0,1) = sda2, (hd1,2) = sdb3, etc. This is not a part of the file system, / or otherwise. It is part of the partitioning layout and is unrelated to the file system on that partition.
To look at it in a very simplistic way, you can think in terms of the BIOS points to a bootable kernel,(grub on the boot drive), which points to a menu, which points to a bootable kernel. This last bootable kernel can be either a Linux kernel, a Windows loader, or another bootable grub kernel located on the MBR of another drive or another partition, virtual or real. This chain can continue until the booted kernel is smart enough to bring up the system.
If you want more in depth information you can look at the thread below beginning with the post referenced.viewtopic.php?f=46&t=10043&p=63253&hilit=Fred+gboot+denise#p63253