It was originally a refuge point in the middle of space for discarded materials from the Cybertronian society. Over the eons, the refuge point became a large mass in space. Depending on the storyline, it has been depicted as either a traditional spherical planet or as an elongated slab. (In the original movie art and storyboards, the Planet of Junk was intended to be a spherical planet, with several rounded slabs seeming to rise off of its surface. In the end, only the "northernmost" rounded section of the planetoid was retained on film.)
Junkions are regarded by many other races as being slightly to extremely insane, depending on the race. They also have a rather extreme xenophobic streak due to their isolation on Junk, and are naturally put off by and wary of strangers, even going so far as to assault the Autobots when they first crash-landed on their planet.
The Junkions have become allies of the Autobots. (Oddly, their eyes are red, which is usually associated with Decepticons in the cartoon.)
Their only exhibited alternate mode is a motorcycle; when they attack they attack en masse, organised into pairs with one in motorcycle mode and the other riding in robot mode to provide firepower. Junkions are superb technicians, capable of assembling all manner of functional machinery from junk. They also seem to have an affinity with mechanical garbage, allowing them to manipulate it and move it telekinetically. These two abilities combine to make Junkions very durable; Junkions are built out of scrap and are naturally very ramshackle, but if their parts are scattered then they can reassemble themselves almost instantly, repairing and reattaching broken off parts and drawing replacement parts from nearby sources (other disabled Junkions, slain Transformers, junkyards, etc.). As Springer says: "It's not hard to knock 'em down, it's getting 'em to stay down that's the trick!". On Junk they are nearly invincible, as they would need to be utterly vaporised to stop them. As Wreck-Gar says "We check in, but we don't check out."
The only notable Junkion is their leader, Wreck-Gar. His voice is provided by Eric Idle in the movie and Tony Pope in the television series. The rest of the Junkions voices were provided by Frank Welker.
Online retailer e-Hobby recolored the toy for the Autobot Hound in a primarily light brown/sand colored deco into the Junkion mercenary, Detritus, who, unlike the other natives of his homeworld, is not aligned to either the Autobot or Decepticon factions. However, this character never appeared in the animated series.