Often times in fanfiction, authors use certain abbreviations or phrases that may be confusing to new users. Here is a basic list. The terms can apply to all fanfictions but the example are Harry Potter related.
AU: AU is an abbreviation which stands for "Alternate Universe". Usually AU fanfictions disregard canon. An example of AU story would be where Harry's parents lived and Hermione was adopted by the Malfoys.
Bashing: A term used in fanfiction when the author puts down, says derogatory things about, or emphasizes the negative traits of a character or group of characters. For example, in Ron-Bashing fanfictions, Ron is portrayed as a jealous scumbag who only cares about money and is only Harry's friend because someone (usually Dumbledore) is paying him (often with the contents of Harry's Gringotts vault).
Canon: Canon refers to the original story. For example, a canon fanfiction would include an orphan Harry and a rich Draco.
Crack (fic): A crack fic is fanfiction that purposely ridiculous or random implying that it could have only been written while under the influence. Crack fics are generally humorous and don't make much sense. An example of a Harry Potter crack fic would be a female Ron who runs away to elope with Hedwig while Blaise Zabini is crowned King of the Wizarding World.
Crossover: Crossovers are fanfictions that include elements from multiple fandoms. An example of Harry Potter/Twilight crossover may have Hermione moving to Washington state and attending Forks High School where she meets Vampires.
Dark: Dark fanfictions are those in which the characters, content, or both are much heavier or "darker" then their original cannon counterparts. In Harry Potter fandom, "dark" can also refer to characters literally joining the dark side i.e. Voldemort's Death Eaters or using excessive dark magic. Darker themes may include rape, abuse, death, or war.
Drabble: A drabble is a short fanfiction of debatable length (usually accepted to be about 100 words, but also known to have as little as 50 words or as many as 500 words) that usually has no real plot.
EWE: EWE stands for "Epilogue, what epilogue?". In EWE fanfiction, the epilogue of the last Harry Potter book would be disregarded.An example of this would be a fanfiction where after the final battle, Harry runs away and marries Parvati Patil.
Fandom: The term fandom is used to refer to the community surrounding a TV show/book/movie/etc.
Fanon: Fanon refers to a common fact, plot, or character element that is not canon (not mentioned in the books, movies, or by JKR) but is still generally accepted as fact by fans. One Harry Potter fanon fact is that Dreamless Sleep Potions are addictive.
Femslash: Femslash implies that one or more relationship is lesbian i.e. Female/Female.It is also referred to as "femmeslash", "F/F", and "yuri". Compare with "slash" and "het". A femslash Harry Potter relationship may include Ginny/Luna or Daphne Greengrass/Morag MacDougal.
Fluff: Fluff fics tend to lack a plot, and be cute and romantic. There is generally no angst in "fluffy" fanfictions. A fanfiction can also have "fluffy" scenes or certain moments when character exhibit fluff. Fanfictions that include fluff are prone to happy endings.
Het: Het stands for heterosexual and implies that there is at least one heterosexual relationship i.e. male/female.It is also referred to as M/F. When listing a het pairing, the name of the male usually comes first. Compare with "femslash" and "slash". An example of het pairing would be Ron/Hermione or Harry/Ginny.
IC: IC is an abbreviation which stands for "In Character" implying that the character is acting as he or she would in canon. Compare with "OOC". An "IC" Hermione would be intelligent.
Lemon: Lemon fanfics are usually very rude and sexual, as well as not very age appropriate for readers under the age of 18. They are mostly smut fanfics.
Mary Sue: Mary Sue is a term used to describe a completely unrealistic original character. Mary Sues are known to be the prettiest, smartest, funniest, and most powerful characters that EVERYONE falls in love with and have very long and implausible names. Most Mary Sues have no flaws. Sometimes, Mary Sues are self-inserts with a load of improvements. Male Mary Sues are known as "Gary Stus" or "Marty Stus".
MPreg: MPreg stands for male pregnancy and include males who are capable of bearing children. MPreg fanfictions usually (but not always) include a slash pairing and are AU (since in real life, men most often can't get pregnant). 
OC: OC stands for original character. It is a character which is not part of the original canon and was created by the fanfic author. Mary Sues and Self-Inserts are examples of original characters.
OFC: OFC stands for Original Female Character. Compare with OMC and see also: OC.
OMC: OMC stands for Original Male Character.Compare with OFC and see also: OC.
OOC: OOC stands for "Out of Character" implying that the character is not acting as he or she would in cannon. Compare with "IC". A nice and sweet Dolores Umbridge is OOC.
OTP: OTP stands for "One True Pairing" and is used by authors to describe their favorite (or one true pairing) in a certain fandom. See also: "Pairings". An example would "My OTP is Harry/Hermione. Hermione was always there for Harry and I feel like they are perfect together,".
Pairings: Pairings refer to the main romantic relationship. Pairings can be het, slash, femslash or any combination of such. Pairings are identifies by a slash in between the names of the characters in the relationship (i.e. Neville/Luna) or combination of the character's names (i.e. Drarry is Draco and Harry).
PWP: PWP can stand for two things: "Plot? What Plot" or "Porn Without Point/Plot"."Plot? What Plot?" fanfictions have no plot and may be simply drabbles or vignettes. "Porn Without Point/Plot" fanfictions also usually lack a plot and its only content usually strictly sex. Most "Porn Without Point/Plot" are rated M or higher. See also: Smut.
Slash: Slash implies that one or more relationship is homosexual i.e. Male/Male.It is also referred to as "M/M", and "yaoi". In a slash pairing, the names are listed in the order of "dominance". The dominant male goes first. If there is no "dominant" male, then the names are listed in alphabetical order. Compare with "femslash" and "het". A slash Harry Potter relationship may include Ron Weasley/Dean Thomas or Harry Potter/Theodore Nott.
Self-Insert: A self-insert is a kind of Original Character where the author of the fanfiction inserts themselves into their story. Most self-insert fanfictions are AU, although there are some that follow canon when based on games.
Smut: Smut refers to a story or scenes in a story that contain graphic sexual content.
Shipping: A term used to describe the pairing of two (or sometimes more) characters, usually in a romantic way. "Ship" can also be used as a verb as in "I ship Draco/Pansy." meaning that you use them as a pairing.