Here's a bit more information on how stories and tags are categorized and managed.
yuuo-lj or zanne-lj: fics that were originally posted on LJ.
Character name: If there's just a single character name, with or without a pairing tag, it means that's the only character actively present in the 'fic. The qualifying rule for that is the character must be alone* through most, if not all of the 'fic. Being beside someone who's asleep, for example, doesn't count.
* random NPC-type characters don't qualify. The character could be intermingling and chatting up a storm with cast extras and local color, but would still qualify for a that-character-centric tag.
Rating tags: If, for example, you want to look for only NC-17 stuff, or only want to read g-rated stuff, that's one way to sort them.
Smut: All smut stories are NC-17, but not all NC-17 stories are smut. Something very graphically violent may be NC-17, as well as stories with heavily implied but non-explicit sex scenes. Smut tags indicate stories which are more or less just sex, and relatively explicit at that.
z-commname/y-commname: some 'fics were written for theme communities. The first letter prefix denotes whose claim the tag links to.
Drabble: Catch-all tag for half-drabbles (50 words precisely), drabbles (100 words), double-drabbles (200 words), etc. To qualify for the drabble tag, all fics must be in exactly 100-word increments (half-drabbles notwithstanding.)
Flash Fiction: proper literary term for any story 499 words or less.
Short Short: proper literary term for any story between 500 to 999 words.
Short Story: proper literary term for any story between 1,000 to 16,499 words.
Novella: proper literary term for any story between 17,500 to 39,999 words. (For further reference, 40,000 words on up is a novel.)
Not Previously Released: A fic that was written in the past, but has not appeared publically anywhere on LJ before now.
Fic name tags: Any fic with three or more chapters, or part of a specific series of 'fics (such as a trilogy) has its own title tag to make it easier to locate the different pieces, especially if it gets posted as a WIP.
FMA-Bluebird's Illusion: Fics based off the Chinese fangame. Also related to Pride!Ed tags.
Ishbal: Any fic, either in direct setting or flashback, is pre-series and set around the time of the Ishbal Rebellion.
FMA: any FMA fanfiction, either manga or anime canon. If there's an -anime or -manga tag, it breaks it down further specifying which canon.
[name] x [name]: a pairing tag (ex. envy x ed; winry x sheska, etc.)
[name] and [name]: a gen tag (ex. ed and al; ed and roy, etc.)
Royai: Roy x Riza pairing fanfiction
Elricest: Ed x Al pairing fanfiction
Heid, or Alfons: Refers to Alfons Heiderich, appearing in the FMA:CoS movie.
Preseries/Postseries: Anything set before the events in episodes 1 and 2 (Lior) in the FMA anime for preseries, and anything set post-episode 51 for postseries. This includes movie setting 'fics. Preseries tags are not necessarily exclusive to FMA anime canon, but postseries tags are.
Other: Characters and/or pairings that are not likely to be written again, or often enough to warrant their own tags.
Jossed: Fanfics that were written prior to canonical information being released which rendered them null and void. Typically manga!FMA canon here. (For more information on the definition of 'Jossed', click here.)