Triton was Discovered by Lassel in 1846, 
only a few weeks after the discovery of Neptune. 

The only spacecraft that ever visited Triton was Voyager 2, which brought back almost all the information we now have about the satellite.

Triton's axis of rotation is unusual, tilted 157 degrees with respect to Neptune's axis (which is in turn inclined 30 degrees from the plane of Neptune's orbit). This adds up to an orientation with respect to the Sun somewhat like Uranus's with polar and equatorial regions alternately pointing toward the Sun. This probably results in radical seasonal changes as one pole then the other moves into the sunlight.

Triton's density is slightly greater than that of Saturn's icy moons. Triton is probably only about 25% water ice with remainder rocky material.
   Voyager found that Triton has an atmosphere, albeit a very tenuous one, composed mostly of nitrogen with a small amount of methane. A thin haze extends up 5-10 km.
   The temperature at the surface of Triton is only 34.5 K (-235 C, -391 F), as cold as Pluto. This is due in part to its high albedo which means that little of the Sun's meager light is absorbed. At this temperature methane, nitrogen and carbon dioxide all freeze solid.

There are very few craters visible; the surface is relatively young. Almost the entire southern hemisphere is covered with frozen nitrogen and methane.

   There are extensive ridges and valleys in complex patterns all over Triton's surface. 
These are probably the result of freezing/thawing cycles.

The most interesting (and totally unexpected) features of this unusually interesting world are the ice volcanoes. The eruptive material is probably liquid nitrogen, dust, or methane compounds from beneath the surface. One of Voyager's images shows an actual plume rising 8 km above the surface and extending 140 km "downwind"