Triton is the largest moon orbiting Neptune. According to readings from Voyager, the moon most likely did not originate from Neptune but was a lost mass that was captured by the huge planet's gravitational force. It's moving in a retrograde orbit which means it spins in the opposite direction of the planet's rotation.
Triton is about three-fourths the size of the Earth's moon. It also has about the same size, density, temperature and chemical composition as Pluto.