What is hd0 and sda/sdb in Linux?

When GRUB probes devices it makes a device.map file, you can find this in the same directory as your grub.conf. It will look similar to this:

(fd0)   /dev/fd0
(hd0)   /dev/sda
(hd1)   /dev/sdb
(hd2)   /dev/sdc

You are free to modify this as you see fit any time. Read more here.

UPDATE (Oct. 21, 2015) : This answer was originally written for GRUB 0.9x which is not under active development and is now referred to as GRUB Legacy. If you are using a version of GRUB 2.x you want this page with details and advice on device mapping.

I believe that

sda = hd0, 
sdb = hd1, 

so on etc. etc. sdc = hd2 When it says sda1 that would be (hd0, 0) and sda2 (hd0, 1) and sda3 (hd0, 2) and sdb1 (hd1, 0) so on and so forth...