General State of Being:
A Rot is a Wraith or Spectre who was flung into a dead, human body as a result of the 6th Great Maelstrom. For some of them this is a terrible setback. For others it's the best thing that could have ever happened, even if they're trapped in their own corpse, or someone else's.
The first Rots were sent on through the Shroud at the outbreak of the Maelstrom. More have come through in the time since then, carried on back through a sudden squall or other weird contusion of the Storm. Some say that a select group of Wraiths has learned how to send their number through the Shroud without needing to risk the Maelstrom winds, but such claims have yet to be proven.
Rots sometimes awake to find themselves in their own bodies, but sometimes are to be found in someone else's. There does not seem to be a correlation between whether the body was a Fetter, or not, for them to have returned in their own body. Some have said that the distance to their body at the time of their being taken by the Storm may play a part in whether they're placed in it or not, but, again, this is difficult to prove.
What They Are Not:
Rots are not Risen. They do not reverse the process of decay when they awake within the body, and they have no need of a Conduit. Likewise, the Shadow needs no placating to allow this to happen - at least, so far as Rots know. They have no access to Vampiric Disciplines, and their Shadows are quite capable of sending them into Catharsis.
Likewise, Rots are not Shamblers. The rotting, nearly-mindless monstrosities that plague the living seem fixed in their decay, and not nearly as resilient as Rots or Risen. However, there is a precedent for Rots who fall immobile for too long to become Shamblers.
Rots need dead bodies, and the only requirement for a suitable corpse is that its brains are more or less intact - if a little soupy from decay. There also seems to be a preference for bodies that are themselves more or less intact, though the absence of badly-damaged Rots might simply be due to those bodies' inability to get out of the grave.
Once inside the body, the Wraith's own physical characteristics are impressed upon it. A weightlifter in the body of a literal 90-pound weakling can pick up adversaries and toss them about the room. Conversely, a 90-pound weakling cannot use the body of a weightlifter to an advantage.
For the Rots, the bodies they inhabit are their "lifelines": if it is destroyed, they will be sent back into the Shadowlands - most likely to a Harrowing. It is not easy to destroy a Rot, especially when it's in an advanced state of decay, but it's not impossible, either. Rots must also be careful to not overtax their bodies, or else they may do damage to them.
The wraith's control over the body transcends mere physicality. Rotten limbs still move, in spite of the loss of muscles or tendons, and severed nerves still feel. Likewise, a Rot with no eyes can see, and the senses of hearing, smell and taste are still reported in spite of damage to - or the complete lack of - the sensory organs in question. A Rot with no mouth cannot make herself heard in the Skinlands, but those in the Shadowlands will hear the Wraith speak.
Perhaps even more bizarre than that, a Rot is able to control detatched limbs - moving a lopped-off hand, for example - so long as they're within sight.
While a Rot, the Wraith is generally cut off from the Shadowlands and Tempest, and cannot sense them, or their inhabitants. Some Rots show an aptitude for being able to look back where they came from, or at least hear what goes on there, but most of them do not have that option. They retain their Lifesight, Deathsight and heightened senses, but these are not as good as they used to be.
The Rot cannot willingly leave her new body. She can squeeze little pieces of herself outside of the corpse, as with certain Pandemonium or Usury Arts, but these are lost to the Art. She cannot extend her Corpus out to control or change other things, as with Inhabit or Flux.
Their Shadows are still active, and quite willing to take advantage of the new situation as opportunities present themselves. In fact, Shadows have a certain edge: when a Rot's physically inactive, her shadow has an easier time employing Thorns, or initiating Catharsis. This, coupled with the fact that too much activity may damage a Rot, contributes to a short "life" expectancy for these Wraiths.
Harrowings are poison for Rots: should they fall into one, they leave the body and go to the Labyrinth, returning to the Shadowlands or Tempest when they come back from it.