A trans-neptunian object is any object that orbits the sun from a distance farther than Neptune. Ok, so you're saying that Pluto should then be a TNO, well it probably would have been if it were discovered recently instead of in the early 20th century. These objects occupy space in the outer solar system that is divided into the Kuiper Belt and the Oort Cloud.

The astronomers Jewitt and Luu discovered the first of TNO in 1992. The community of astronomers decided on a new category for this object because surface reflection data revealed no evidence of a comet's dust tail. Since 1992, astronomers have located 578 trans-neptunian objects. The most famous of TNO is Varuna, which has a diameter of 800 km, or roughly 1/3 the size of Pluto. Varuna was discovered in 2000. However, Varuna is not the largest of the TNO. In 2001, astronomers pinpointed an object nearly half the size of Pluto with a diameter of 1100 km.

Plutinos and cubewanos are two classes of trans-neptunian objects. Plutinos have a similar orbit to Pluto because a gravitational resonance from Neptune works to stabilize the objects. Cubewanos have more unique orbital paths because they do not require the resonance of Neptune to remain in orbit around the sun.

Jewitt and Luu describe TNO as being relics from the accretion disk of the sun which circumnavigated the entire solarscape during an earlier stage of the sun's life. Recent spectrographic imagery suggests water that may exist on Pluto and Varuna.

Update: March 14, 2004 - NASA announced the discovery of the farthest TNO known to date. It is thought to be about 3/4 the size of Pluto, and is named Sedna.