I understand your confusion. Sometimes these terms are used as if they are interchangeable, but they are not.
Let's assume you are talking about a physical disk drive as the field replaceable unit (FRU).
Hot Swap means that you can remove the drive and replace it with another drive without significant interruption to the system. In case of a mirrored disk environment, the system should re-synch with the new drive to reestablish the mirrored pair. In the case of a RAID configuration, the system performance may be degraded until the drive is replaced and the checksum data is spread across the new drive, but again, there is no significant interruption to service.
Hot Plug is a little tougher. Hot Plug typically means that you can add a new FRU (a disk drive in our example), but you can not remove the FRU without taking some sort of outage. I say that Hot Plug is tougher in the sense that you need to be careful and not interpret Hot Plug as having Hot Swap capability.
Hope this helps.
This was first published in July 2001