Rating
G - Stories suitable for anyone, including children.
PG - Mild language, violence, and/or limited sexual content.
PG-13 - Stronger language, violence, and/or sexual content; unsuitable for children under 13.
R - Strong violence, harsh language, and/or sexual situations; usually for people over 17.

Categories
Action/Adventure - Stories with an emphasis on action.
Alternate Universe - The story changes certain facts in the known universe so that it differs from the one we know from the books.
Angst - Mostly emotional hurt/anguish and tense situations.
Drama - Dramatic stories, intense conflicts, sometimes sad things happen.
General - A story that doesn't fit anywhere else.
H/C (Hurt/Comfort) - Stories where a character is hurt (physically or emotionally) and another character comforts him.
Humour - Stories with funny elements.
Missing Scene - Stories that tell us what we didn't see in the books. Often stories that fill the "gaps", or different points of view.
Series - The story is part of a series or has a sequel/prequel.

Main Characters
Harry Potter
Ron Weasley
Hermione Granger
Ginny Weasley
Draco Malfoy
Sirius Black
Remus Lupin
Peter Pettigrew
James Potter
Lily Potter
Severus Snape
Rubeus Hagrid
Albus Dumbledore
Lucius Malfoy
Tom Riddle
Voldemort
Dementors
Original Character
Other Character

Era/time frame
Tom Riddle time - Stories that are set during the time of Tom Riddle's school years.
MWPP time/VoldeWar I - Stories that are set during and after the time of Moony, Wormtail, Padfoot and Prongs's school years.
during the books - Stories that are set in the time frame of the books (i.e. Harry's childhood up to 7th year).
after the books - Stories that are set after Harry's 7th year. (none yet)
unspecified - The time frame is unknown or not important.