The planet of Junk (sometimes also referred to as Junkion) is a planetoid which was first introduced in Transformers: The Movie. It is the home planet of the Junkions, a subgroup of Autobots who transform into motorcycles.
It was originally a refuse point in the middle of space for discarded materials from the Cybertronian society. Over the eons, the refuse 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. (low gravity?) In the end, only the "northernmost" rounded section of the planetoid was retained on film.)
Junkions
Eventually an esoteric group of Transformers known as Junkions inhabited Junk, and a unique culture formed on the planet, which is close enough to receive television broadcasts from Earth with only a decade or two of lag. As a result they only speak in phrases from commercials and TV shows, given this, Junkions can be somewhat hard to understand; they tend to talk in somewhat of a mish-mash of catch phrases and sound bites (referred to in the film as "talk[ing] TV").
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 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 teams 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 a telepathic affinity with mechanical garbage, allowing them to manipulate it and move it telekinetically. These two abilities combine to make Junkions very endurable; 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.
Interestingly, the background soundtrack to the battle on the Planet of Junk is "Weird Al" Yankovic's Dare to Be Stupid. The song is littered with TV catchphrases and is actually a rallying song encouraging others to "dare to be stupid." It almost seems to be a speech encouraging the Junkions to join in the fight.
Devil's Due Comics
Serpentor learns of the Planet of Junk in Devil's Due comics.
In the third volume of the Devil's due comics the android Serpentor downloaded information on the history of Cybertron from Soundwave. Mentioned among that information was the Quintessons, Alpha Trion, Megatron and Soundwave, the planet of Junk, a warrior named Optimus Primal and the Autobot Matrix of Leadership.