Hi Mr,
Unmount - It unmounts the VMFS filesystem from the ESXi host.
"When you unmount a datastore, it remains intact, but can no longer be seen from the hosts that you specify. The datastore continues to appear on other hosts, where it remains mounted"
Detach - It will disconnect the host connectivity with the Storage system. Though the LUN is still visible on ESXi host until removed from the Storage side.
It is same like ejecting the device. Example, when you do the USB device eject. You click on the small Icon and "Eject my USB" . Though the USB is still connected until you pull it out.
Thanks,