[3] Species in Poecilia are called mollies (e. g. P. sphenops) or guppies (e. g. P. reticulata) depending on body shape.

[7] Poecilia refers to the Greek word poikilos, which means "variegated" or "speckled", an allusion to the coloration and pattern of the type species, as well as that of and other related fishes, such as Fundulus heteroclitus which Bloch & Schneider classified in this genus.

The most commonly kept species are guppies (P. reticulata), mollies (P. sphenops or P. latipinna), and Endler's livebearers (P. wingei).

[9] They are easy to sex as males have a prominent gonopodium, a modified anal fin used to inseminate females.

They mature quickly and breed readily, with females giving birth to a dozen or more fry every month.