Neptune is a blue Gas Giant like Uranus and is the furthest planet from the Sun, at around 30 times further away than the Earth is. The gravitational pull on the surface of the planet is one of the strongest in the Solar System, with only Jupiter having a stronger pull.
Neptune was first 'discovered' through observations of the orbit of Uranus, which looked like it was being pulled by another planet, therefore it was found through a mathematical formula (by Alexis Bouvard), rather than an observation!
Like Uranus, it contains a higher proportion of "ices" such as water, ammonia and methane, and it is the methane that gives Neptune its blue colour. The white streaks you see are high level cloud layers that indicate active weather systems in the planet's atmosphere. Neptune's winds are the fastest known on any planet, with speeds approaching 2000 km per hour.
Neptune has at least a dozen small moons and one large one called Triton.
Voyager 2 is the only spacecraft to visit Neptune back in 1989, it took some images of the moons as it flew past including the mosaic of Triton to the left.