Not much is known about the Nemean Lion, other than the story of Hercules and the first of his twelve trials. Most mythologists believe that the Nemean Lion represents the constellation of Leo. For a little-known monster of mythology, that's not a bad legacy.
The Nemean Lion is named as such because he was a gigantic, ferocious lion with impenetrable skin that lived in a cave in the land of Nemea. Interestingly, while most other monsters in Greek mythology have a name, the Nemean Lion doesn't seem to. I mean, what did they call him before he moved to Nemea? "The Lion"?
As the son of the god/monster, Typhon, and his lady/snake bride, Echidna, this makes this fearsome feline a sibling of some of mythology's greatest monsters: The Sphinx, Cerberus, Hydra, Chimera, and others.
The goddess Hera has essentially played "Godmother" to most of Greek myth's great monsters, including the Lion of Nemea. It was Hera who first begged Gaia and Tartarus to create the god/monster Typhon, who happens to be the Lion's father. Hera, angry at Zeus, sent the lion to Nemea to live, where there just so happens to be a shrine to Zeus. Thus, anyone wanting to worship Zeus would have to get through a gigantic, almost indestructible lion first.
The Lion of Nemea's story is short, but important. He was the first of the trials of Hercules, child and brother to some of legend's greatest monsters, and possibly lives on in a constellation. He was said to be adored by Selene, the moon goddess, as well as Hera, who is said to have placed him among the stars.