Hares and jackrabbits are mammals belonging to the genus Lepus. They are herbivores and live solitarily or in pairs. They nest in slight depressions called forms, and their young are able to fend for themselves shortly after birth. The genus includes the largest lagomorphs. Most are fast runners with long, powerful hind legs, and large ears that dissipate body heat. Hare species are native to Africa, Eurasia and North America. A hare less than one year old is called a "leveret".

Members of the Lepus genus are considered true hares, distinguishing them from rabbits which make up the rest of the Leporidae family. However, there are five leporid species with "hare" in their common names which are not considered true hares: the hispid hare (Caprolagus hispidus), and four species known as red rock hares (Pronolagus). Conversely, several Lepus species are called "jackrabbits", but classed as hares rather than rabbits. The pet known as the Belgian hare is a domesticated European rabbit which has been selectively bred to resemble a hare.

Hares are swift animals and can run up to 80 km/h (50 mph) over short distances. Over longer distances, the European hare (Lepus europaeus) can run up to 55 km/h (35 mph). The five species of jackrabbits found in central and western North America are able to run at 65 km/h (40 mph) over longer distances, and can leap up to 3 m (10 ft) at a time.

Normally a shy animal, the European brown hare changes its behavior in spring, when it can be seen in daytime chasing other hares. This appears to be competition between males (called bucks) to attain dominance for breeding. During this spring frenzy, animals of both sexes can be seen "boxing", one hare striking another with its paws. This behavior gives rise to the idiom "mad as a March hare". This is present not only in intermale competition, but also among females (called does) toward males to prevent copulation.

Hares, like all leporids, have jointed, or kinetic, skulls, unique among mammals.

Hares are generally larger than rabbits, with longer ears, and have black markings on their fur. They have 48 chromosomes, while rabbits have 44. Hares have not been domesticated, while some rabbits are raised for food and kept as pets.

Some rabbits live and give birth underground in burrows, with many burrows in an area forming a warren. Other rabbits and hares live and give birth in simple forms (shallow depression or flattened nest of grass) above the ground. Hares usually do not live in groups. Young hares are adapted to the lack of physical protection, relative to that afforded by a burrow, by being born fully furred and with eyes open. They are hence precocial, able to fend for themselves soon after birth. By contrast, rabbits are altricial, being born blind and hairless.

Easily digestible food is processed in the gastrointestinal tract, expelling the waste as regular feces. For nutrients that are harder to extract, hares, like all lagomorphs, ferment fiber in the cecum and expel the mass as cecotropes, which they ingest again, a practice called cecotrophy or refection. The cecotropes are absorbed in the small intestine to use the nutrients.