The terms saber-toothed cat, sabertooth, and saber-toothed tiger describe numerous species, mainly in the families Felidae (subfamily Machairodontinae), Barbourofelidae, and Nimravidae, but also including two marsupial families, that lived during various parts of the Cenozoic Era and evolved their saber-toothed characteristics entirely independently. They are most known for having maxillary canines which were, in some species, up to 20 cm long and extended down from the mouth even when the mouth was closed. Saber-toothed cats were generally more robust than today's cats and were quite bear-like in build.

source:en.wikipedia.org