For the majority of people, a leopard is a leopard. Sure there are quite a range of subspecies, but one generally looks rather like the other. Indeed the only significant difference people often notice is one that has not justification – that of the melanistic leopard (or black panther)
In order to disturb wild leopards as little as possible, the study took DNA samples from museum held leopard specimens all over the world.
African Leopard 
Asian Leopard
Perhaps the most startling discovery, was the huge differences between Asian and African Leopards – these differences are greater than the difference between Polar bears and American Grizzly bears.
What is particularly interesting is how fast this divergence seems to have grown up. Polar bears and grizzly bears are separated by roughly 1 million years, Neanderthals and humans however are separated by 500,000-600,000 and this is roughly the time when African and Asian leopard species split.
The differences are rarely encountered between two different branches of the same species, this significant a difference is not always visible between different species. By testing recent Leopard samples, the research was also able to rule out human impacts – thankfully while we have pushed a handful of leopard species to extinction, our impact is not yet showing clearly in their genetics.