Here is a small glossary of fanfiction terms to help you if you're new (and amuse you if you're already in the know).
Canon
Crack!fic
Crossover
Fandom
Fanon
Femmeslash
Manon
Marauders
Mary Sue
MWPP
Next Generation
Potterverse
Ship
Shipping
Slash
Trio
Canon – what the original author has to say on the matter. In the Potterverse, canon is made up of the seven Harry Potter books, Tales of Beedle the Bard and the prequel drabble, all authored by J.K.Rowling. Whether or not interviews are considered canon is a long-raging debate, especially seeing as Ms. Rowling contradicted herself a few times (which, considering the fact that she was answering questions while the books were still being written, is only to be expected).
Crack!fic, crack fic, crackfic – a fanfiction with a premise too ludicrous to be accepted by common or uncommon sense. Very often the result of a humour fic gone wrong, or of an attempt at self-insertion. The genesis of the term is believed to derive from the concept that some ideas of crack fics sound like their author was using the drug crack at the time.
Here at DungeonLight we do not accept crackfics, however clean and sober you might've been when you came up with them.
Crossover – a fanfic featuring characters of different fandoms, whether their meeting is arranged by placement of one group in another's world, or the two fandoms being sides of one and the same world. For example, a story about Luke Skywalker encountering Earth at some point during his adventures, and falling in love with Hermione Granger.
Here at DungeonLight we do not accept crossovers.
Fandom – among other meanings, any world or universe that generates fan following and fan art. The world may have a canonical name (like Terry Pratchett's Discworld or Pierce Anthony's Xanth) or a fan-given name, often having to do with the title or main character (like the Potterverse being named after the title character of the Harry Potter series, or Robert Jordan's Wheel of Time world being referred to as 'Randland', after Rand al'Thor, one of the central characters of the series).
Fanon – a concept that was not stated in canon, but so widely accepted by fans that it's considered 'almost canon'. An example of fanon is the James Potter's Glasses debate. It is commonly accepted that James, like Harry, has been wearing glasses from an early age. However, the only canon mention of James wearing glasses refers to his image in the Mirror of Erised, where he looks old enough to be eleven-year-old Harry's father (contrary to the popular belief, Chapter 38 of Harry Potter and the Order of the Phoenix, Snape's Worst Memory, does not explicitly state that James wore glasses at the age of 15/16, only that he looked a lot like Harry).
Another example of fanon are fan-invented words, such as 'to owl someone' instead of 'to send an owl to someone', or 'to Floo somewhere' instead of 'to go somewhere by Floo network'. While both expressions make sense from a language point of view, neither of them was used in canon 'as is'.
Here at DungeonLight we will not reject your story for fanon, provided it does not contradict canon.
Femmeslash – female/female relationship.
Manon - 'movie canon'. Examples of manon include Hermione with make-up, Lucius with a cane, and a countercurse to Sectumsempra that roughly translates to 'heal wound'.
Here at DungeonLight we are progressively annoyed at how much the movies influence the perception of concepts and characters in the Potterverse, and WILL reject your story for manon. We believe in fanfiction based on the creation of J.K.Rowling, not Warner Brothers.
Marauders, the Marauders – James Potter, Sirius Black, Remus Lupin, Peter Pettigrew.
Mary Sue – nowadays the term is commonly used to describe a character who is far too perfect to be true. Indications of a Mary Sue are extremely beautiful looks, exotic-sounding names, outstanding skills and general superiority to the rest of whichever race they are a member of. A male version of Mary Sue is called Gary Stu. If your characters get accused of being any of that, check them for a number of dimensions (chances are more need to be added).
MWPP – Moony, Wormtail, Padfoot and Prongs. See also, the Marauders.
Next Generation, Next Gen – children of Harry and his generation. Often includes Teddy Lupin, despite him being somewhat older.
Potterverse – the universe where the events of the Harry Potter books take place and, respectively, the setting of all non-crossover Harry Potter fanfics.
Ship – short for 'relationship'. Can be used to refer to any pairing or couple, whether heterosexual or homosexual.
A ship is normally described as Name/Name (Ron/Hermione, Harry/Ginny). The order of the names is not necessarily consequential. The male name often gets put first by force of habit.
Sometimes, however, the order of the names in a ship description may indicate the requited-ness (or lack of) of a relationship. By that logic, one could say that Severus/Lily is a canon, if unrequited, ship, as he had loved her. If, however, one reverses the order of their names in the ship description, one may be called up on that for being canonically inaccurate.
Some ships have titles made by moulding the names of their participants together directly (Dramione, or Draco/Hermione), referring to their participants indirectly (Wolfstar, or Sirius/Remus) or describing their nature (the Ghost Ship, or Severus/Lily).
Shipping – being in favour of a certain pairing, whether canon or non-canon, and/or writing fanfiction featuring the ship in question. 'Sailing the [name/name] ship' is another term for shipping.
Slash – any homosexual relationship.
Trio, the Trio, the Golden Trio – Harry Potter, Ron Weasley, Hermione Granger.