The internal structure of Neptune is a bit different from those of the other gas giants: an atmosphere of gas (mainly hydrogen, helium and methane) overlays a huge ocean of liquid water and ammonia. This ocean makes up most of the mass of the planet. Like the other gas giants Neptune is also believed to have a solid core made of iron, nickel and silicates (rock). The conditions in the centre of Neptune are quite extreme: a pressure of 7 million bar causes temperatures to rise above 5000 K, which is still much lower than the 40 million bar and 36000 K inside the super-massive planet Jupiter.
Like all the other gas giants, Neptune has a ring system, but it's so faint that it hadn't been definitely confirmed before 1989 when Voyager 2 had its flyby.