The leopard - or "chui" as it is known in Kiswahili - is the smallest of the four big cats (lion, tiger, jaguar, leopard) in the genus "Panthera". In fact, the term "panther" is often used as an umbrella term to refer to any species in this genus (such as lion, cougar, jaguar). This is why "black panther" is used to describe either a leopard or a jaguar that is "melanistic" (has a skin that is mostly of the dark melanin pigment making the animal look black or very dark as a result). If you want to see a black panther - in this case black leopards - then the Aberdares region of Kenya is a place to go...
There are various subspecies of leopard living in Africa, Asia and the Middle East, but this page is about the African subspecies (Panthera pardus pardus).
The leopard is a "stocky" cat: it has shortish legs and a powerful body with a big head. This clearly distinguished it from the much more slightly built and gangly-looking cheetah, and the two species also have very different spots, as you can see from below:
Leopards will pretty much eat anything: from scavenging for beetles and lizards, to hunting much larger prey such as antelopes! Like most cats, they hunt by stalking to get as close to their prey as possible, then swiftly close in with the aim of strangling the prey by latching onto its throat. They are also extremely good swimmers and will also hunt for fish!
Leopards have a massive advantage because, unlike the other big cats, they are great tree-climbers. They will carry their kill - perhaps three times their own weight - up a tree by utilising their especially strong jaw and neck muscles along with their powerful retractable claws. They even sometimes hunt from trees - pouncing down on unsuspecting prey!
In fact, leopards tend to use trees kind of how we use larders! They can't eat larger prey in one sitting so will simply stash it up a tree and return to eat bits of it when they feel hungry! Keeping, and eating, your food up a tree like this is an excellent means of preventing other predators (like lions and hyaenas) from stealing it! If you see a hyaena wandering around at the base of a tree, looking up it longingly, you can pretty much guarantee it is because there is a leopard's food stash up there - perhaps being eaten by said leopard, the hyaena of course hoping for some bits to fall down so it can have a share!
Leopards are usually nocturnal, so they usually (but not always) hunt at night and sleep by day. Indeed, a leopard will not only eat up a tree, but frequently sleep up one too! So well camofluaged are they because of their spots that often the only way you can see one (when out on safari, say) is by catching a glimpse of its long tail dangling down from a branch - if you are lucky! Leopards are highly solitary animals and, apart from mating and, in the Mum's case, looking after cubs, they live and hunt alone.
Females can give birth at any time of the year. The cubs tend to be greyish with less defined spots than their mum and are born after just over a month of gestation. They stay with mum for about two years during which she trains them. It is a dangerous time until they begin to learn tree-climbing and mum will hide her cubs carefully under rocky overhands - anywhere where they can possibly be safe from predation by lions and hyaenas. Leopards can live to around 21 years of age in captivity, perhaps 12 to 17 years of age in the wild.
Leopards do the usual purring and growling, but also make a strange rasping cough-like noise as a kind of "roar" contact call. Leopards have home ranges; the male will have a range that overlaps with that of several females. Ranges are marked with urine and claw marks.
Leopards are very dangerous (unlike, say, cheetahs), not least because they are highly unpredictable. In many ways they are actually more dangerous than lions. Leopards are infamous in captivity for being animals that you must never turn your back on - they can go from appearing to be calm and quiet, to suddenly attacking within the blink of an eye. And when they attack, they really mean it.
Below are some excellent resources about leopards: