She is a giant, twelve-foot long snake and a loyal pet of Lord Voldemort, who as a Parselmouth, is quite capable of communicating with her.
Nagini seems to be the "evil" equivalent to Albus Dumbledore's phoenix Fawkes. Even more so, however, she seems to be Voldemort's spiritual familiar, sharing an intimate bond with her master.
In the sixth book, Dumbledore speculates that Nagini may be one of the six Horcruxes that Voldemort left behind to achieve immortality.