Have you ever formatted a floppy disk? If you have, then it's no different then formatting a hard drive (except a few extra steps when formatting a harddrive). A DOS partition makes the drive "bootable". It allocates space to install windows or any other operating system.
EDIT: A regular partition, as mentioned above splits the drive into multiple logical drives. So it basically cuts it up in whatever sizes you want. So you can make two partitons on say an 80GB drive. Make one partiton of 40GB for linux (different operating system) and make the other 40GB for Windows. Or 60GB for linux and 20GB for windows.
If you want to know how to format a harddrive in a dos prompt do the following:
1. Put a diskette with no valued data in the floppy a: drive.
2. Go to start and open a command prompt, if you don't know what that is go to run and type "cmd" without quotes.
3. Now you have a blinking cursor right? Type "a:" (remeber no quotes)
4. You should now see a: with a flshing underscore.
5. Type format "a:/s". What this is telling it to do is format the disk in the A drive, and the "/s" tells it to write the system files.
6. It should pop up with a message saying "Warning all data will be lost. Continue? Type "y" (for yes) and it should then format. After it's done you have a completley clean disk.
**But remeber this is for a floppy disk. If you want to format a harddrive you must replace the A with whatever letter the drive corresponds to.**
Hope this helps.