To understand the distinctions, one needs to understand the UKC group structure.
As it relates to these breeds, you need to understand the following:
UKC HERDING GROUP - No sub-groups
UKC SCENTHOUND GROUP - Two sub-groups
-TREE HOUNDS - Two sub-groups
As of the 1994 revision the LOUISIANA CATAHOULA LEOPARD DOG was recognized by the United Kennel Club (the most recognizable -working- dog registry in the US) in the HERDING group.
The LEOPARD CUR was recognized by UKC on November 1, 1998. In recognition that the breed was becoming less of a Cur Dog and had essentially become a open trailing Treeing Hound... in no small part because Leopard Cur owners wanted to compete with the Coon Hounds (and not the Curs Dogs) at UKC night hunts, the name was changed to AMERICAN LEOPARD HOUND on May 1, 2008. When this happened, Leopards were moved from the Cur Dogs to the Coonhounds.
So as it stands as of today's date - Dec 12 2008 - the Catahoulas are UKC Herding dogs, and the Leopards are UKC Coonhounds.