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
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
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.