On top of Valorum's list, it is also helpful to describe the relationship between the fic and the original:
- Tone and narrative voice - Is the fic written in the same overall style? For example, A Song of Ice and Fire is known for killing off characters, while Wall-E is not. Does the fic observe similar practices to the original in this and other ways?
- Genre - Did Harry Potter get abducted by aliens with ray guns? Did the Enterprise land on a planet of wizards with pointy hats?
- Setting - Is the fic set in a high school or some other environment that doesn't look like the original?
- Characters - Is a good character evil, or vice-versa? Did someone get aged up or down dramatically? Were any new characters introduced?
- Multiple canonicity - Some stories ("crossovers") mix two or more different fictional universes. There are actually multiple ways of doing that:
- Integrate the universes together, forming a new whole (e.g. Captain Han Solo of the USS Enterprise helps Jedi Padawan Wesley Crusher defeat the evil Romulan empire after the latter destroys Betazed with their Death Star).
- Keep the universes initially separate, but put them in the same physical space (e.g. the Enterprise, with no human crewmembers and no knowledge of Earth, lands on a planet which SG-1 is exploring).
- Have characters from one universe explicitly cross over to the other (e.g. the TARDIS lands in the middle of any non-Doctor Who universe, with the Doctor giving some technobabble about how far away they've traveled or similar).
- Plot - Did events happen differently? Did Greedo shoot and kill Han in Mos Eisley, and derail the entire story?
- Any other striking differences or similarities between the fic and the original.
Obviously, it is also important to mention the original which it was based on. While this may seem trivial, a story about Romulans may look very different if it is based on the Rihannsu novels than if it is based on regular Star Trek canon (i.e. the TV shows and movies), so if the former are in play, you should mention them explicitly.