What is the difference between a leopard and a jaguar?

In general, the jaguar coat has larger rosettes in smaller numbers. The rosettes usually are darker, have thicker lines, and enclose smaller spots. The leopard coat has smaller, more faint rosettes in larger numbers. Leopard rosettes usually don't enclose spots.

There are more distinct differences in the build of these two cats. The jaguar has an overall more muscular build. The body is compact, and the legs are more stocky than the more graceful leopard. The jaguar's head is also more stocky, with a larger looking jaw and an overall more square appearance to the face.

correction to my answer...
"LEOPARD"lives in african continent...NOT the Jaguar..Sorry

Jaguars are native to South America, Central and lower portion of North America (mexico mostly). Their main diet consists of Havelena pigs (aka Pacera pigs).
Jaguars are native to the african continent.
An easy way to tell the difference is that the rosettes on a Jaguar have small black dots in the center. "generally" Jaguars become heavier.

BOTH species can get what's called MELANISM. which is the opposite of Albinism (AKA albino). which is an overproduction of execess presence of Melanin in the system....which then causes more BLACK pigmentation. You can still see the Rosettes.

LONG STORY SHORT.... on Jaguars you can see a dot or dots in the center of the ROSETTES on a leopard NO dots in the center of the Rosettes.

A jaguar inhabits the jungles or rainforests of South America. Leopards prefer the plains of Africa and Asia. Jaguars and Leopards both have black circles (Jaguars are whole, leopard broken) with brown centers and heavyweight boxer or wrestler build, though the jaguar is the larger cat, in fact it is the only cat able to kill with a single bite, having the strongest jaws in the Cat family. And jaguars come in a black version, too, though you can still see the spots underneath the black.

Another huge difference between the leopard and the jaguar is probably derivative from their respective natural environments. The jaguar is a renowned swimmer, and tends to reside in places that have a body of water nearby. The leopard, on the other hand, given its naturally more arid climate in Africa is quite averse to water, except what it uses to drink!

Indeed, both cats have spots and they BOTH can exhibit melanism(black coat). The stockier build of the jaguar makes him less graceful and atheletic than the leopard. The biggest differences are the leopard occupies the eastern hemisphere, the jaguar is here in the western hemisphere; as well as the fact that the jaguar is much larger( largest leopards nearly 200 lbs.----largest jaguars 400 lbs.)

You might wanna add the cheetah to that as well. Apart from habits, these are the main differences in appearance:

Leopard
Lives in Africa & Asia
Spots are broken circles
No dot in circle
Black marking on the backs of ears

Jaguar
Lives in Central/South America
Spots are circles
Dot in center of circle
Belly is white

Cheetah
Lives in Africa & Arabian Peninsula
Spots are solid circles
Black mark from nose to eye