We know that Neptune has at least 14 moons. They are all named after water gods or creatures in Greek and Roman mythology.
The largest of the moons is called Triton (the Greek god of the sea). It was found only 17 days after the planet was discovered in 1846. Triton is big enough to have its own thin atmosphere. It is thought that Triton was passing by Neptune when the Solar System was still forming. It was then captured by Neptune's gravity and became a moon. Most moons are made from the debris left as the planet itself formed. We think that when Triton was captured it made some other moons orbiting around Neptune unstable. These then crashed into each other breaking into tiny pieces. These make the ring system we see around Neptune today.
Psamathe and Neso are 2 of Neptune's other moons. They orbit further away from their planet than any other moon in the Solar System. Apart from Triton, Neptune's moons are all very small. If you added them all together they would not even make 1/100th of the mass of Triton!