Lemurs make up the infraorder Lemuriformes and are members of a group of primates known as strepsirrhines. The term "lemur" is derived from the Latin word lemures, meaning "spirits of the night" or "ghosts". This likely refers to their large, reflective eyes and the wailing cries of some species (the Indri in particular).

The term is generically used for the members of the four lemuriform families, but it is also the genus of one of the lemuriform species, the Ring-tailed Lemur (Lemur catta). The two so-called flying lemur species, known formally as colugos, are not lemurs or even primates. The 4 families of lemurs are split into two superfamilies, containing 99 species and subspecies.

The Cheirogaleidae have a pedal structure similar to the other strepsirrhine families and the haplorrhines, suggesting they split off from the other lemurs first.[citation needed] As such, the Cheirogaleoidea are a sister clade to the Lemuroidea. Lemurs are primates endemic to the island of Madagascar and smaller surrounding islands, such as the Comoros, where they were likely introduced by humans. Molecular genetics indicates that they reached Madagascar after it broke away from mainland Africa, possibly by "rafting" across the ocean on large clumps of vegetation.

While their ancestors were displaced in the rest of the world by monkeys and apes, the lemurs were safe from competition on Madagascar and differentiated into a number of species. These range in size from the tiny 30 gram (1 oz) Pygmy Mouse Lemur to the 10 kilogram (22 lb) Indri. Larger species, some of which weighed up to 240 kg, have all become extinct since humans settled on Madagascar.

Typically, the smaller lemurs are nocturnal, while the larger ones are diurnal.

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