Hard drives show up as icons on the desktop. If you have more than one drive, or more than one partition, then you'd have an icon on the desktop for each drive or partition. If you put in a CD, a CD icon will show up, and so forth.
The thing with the old Mac OS is, the desktop icons are linked with drives rather than profiles, and they will show on the desktop whenever the drive is mounted. So if you have icons on your desktop with the internal hard drive, the same icons will still show up on the desktop if you boot from another drive (like a CD, or another hard drive). Likewise, if you had an external USB drive or whatever, which had its own desktop icons, those icons would show up when you plug in the drive and disappear when you unplug the drive. And if you took that drive and plugged it into a different Mac, those same icons would appear on the desktop of that machine.
It's a little strange to get used to, but it makes sense if you think about it.