Hi Robert,
Steve is basically correct. The upper icon represents the entire disk and the lower, indented icon is the "volume". So Macintosh HD is your "volume".
Note that 149.1 GB LaCie GroupSA is a "hard disk drive" and there are 2 volumes on this disk because it is "partitioned".
Normally when running "Verify" or "Repair Disk" or "Repair Permissions" you would want to choose the volume.
Refer to what Disk Utility tells you before you choose, "Select a disk, volume or image."
Images like "zlock.dmg appear below the divider:
Hope this helps.
-mj
macjack@gmail.com