Panic fandom probably hit its high water mark somewhere around 2007/2008. There's a lot of older fic, and dated fandom moments and some of the stuff is locked or gone even from two years ago.
BUT there is a lot of awesome stuff to go back through, and people are still writing.
First things first, though:
Primers
Again, most of the primers date, but here a few good ones:
* Skoosie's Bandom Primer
Here's 
skoosiepants's Bandom Primer for SGAers. She was mostly a Panic girl at the time I guess, so the main focus is them here. Skoosie is an incredibly prolific bandom author, one of my faves- you might remember her from SGA if you ever were in that fandom. (She also invented with her bare hands, out of thin air, another pairing I love that involves the oldest Jonas brother and the guitarist from the band The Academy Is... that has no reason to exist whatsoever. You ever get so bored you want to read Kevin Jonas/Mike Carden fic, call me. I might laugh at you, but in the rueful way where I've read like every piece of fic for that pairing that exists full stop AND YOU SHOULD TOO.)
* crack_van Primer
Here's the 
crack_van bandom overview, Panic is the second group, but there's a lot of other bandom guys that pop up in all bandom fic so it's a good place to get a general background on anyone you're likely to see in a fic. Sadly, it's old so none of the pictures load anymore, but the written info is pretty good and more up to date in terms of current Panic. Also, the crack_van primer has links in it to all the individual Panic pairing primers, which frankly I am too lazy to link here.
(ETA because I'm a fiddler: Also This one by femmequixotic covers all three groups and is more up to date and has pictures.)
* Timeline! at the Disco by starafar is a really good modern reference for people just getting into this craziness now.
The Band
There is enough in the primers about the origins of Panic, but I will only add that one thing that surprised me when I first got into Bandom was just how young Panic is. (YOUNGER THAN ME. URK. I AM OOOOLD.) They were only 18/19 when their first album was released. Also important, Panic is prooobably the gayest band ever. Maybe since Queen.
Ryan Ross:
(lead guitar, lyricist)
So Ryan is pretty much the kind of kid you'd find smoking on the steps of the English building in college, taking nothing but classes in 20th Century Poetry and Russian Literature and Literary and Cultural Studies. Ryan rolls super intellectual, you could say. He was the driving artistic force for a lot of Panic's stuff- just check out those lyrics, that shit is adorably dense. His fashion sense and makeup choices at the height of Panic's glam-ness were also hilarious- this Ryan Ross makeup primer is a pretty amazing example. Personality-wise he's portrayed as being complicated, spiky, deadpan, your classic artist type with a pretty fucked up family life. :( He's also delightfully unabashedly weird. (RYAN USED TO HAVE A LIVEJOURNAL AND WOULD POST SHIRTLESS PICTURES OF HIMSELF LIKE PEDO BAIT. ILU, RYAN ROSS.) His sense of humor is equal parts sarcastic and thick with irony! Ryan has a lot of feelings but talks in a monotone! He's kind of flake when it comes to taking care of grown up stuff! Oh, and he and Jon Walker are probably stoned aaaaaall the time. (Evidence for this: almost the entirety of Pretty. Odd. as an album.)
Bandom got kind of pissed with Ryan (and Jon) in general when Panic broke up- they gave some bitchy interviews and some other stuff- but things seem relatively okay between everyone now. Also someone on my flist has met him a few times at Young Veins shows and has good things to say about him. I still love you, Ryro.
Ryan, you're the greatest. ♥
Brendon Urie:
(lead singer, also plays guitar and piano)
Oh man, Brendon Urie. (Or Bren or Bden, as he is known.) IDEK. Is he even real. While I relate more to Spencer personally, Brendon is my favorite forever and ever. Skoosie's take on him in her primer is pretty dead on. I will only add a few things. Most importantly, Brendon gives amazing dumbface. (No really, I aspire to be him in the area of dumb facial expressions, he is my role model.) His story is that he comes from a large Mormon family, and by all accounts was fairly lonely in HS. He actually moved out/got kicked out when he was 17 to live on his own so he could continue to be in the band. Fanon also tends to throw in a bunch of 'Brendon got kicked out because he was gay' as well, but you know, fandom. We also like to dwell emotastically on how hard and shitty it was for Brendon to work enough to pay for a crappy apartment AND finish high school AND keep up with Panic, even though to be fair that was only for like 5 months before they got signed. Still, what were YOU doing at 17 besides driving around and hanging out and eating food out of your parents' fridge and whining about AP tests?
Brendon in fandom can seen as being musically multitalented, frenetic, needy, a bit of a dick, dedicated, joyfully dorky, possibly a ho, a hater of wearing clothes, tenacious, annoying, a guy with a huge heart that maybe spends too much time on his sleeve. Oh, and he's a brave little toaster. My favorite Brendon Urie OF ALL TIME is Brave Little Toaster Brendon Urie.
If you want to get an idea of how much of a hyper kinetic muppet Bden is in action, I suggest watching him here in the video for their song Nine in the Afternoon, bouncing around in PJs.
Also, if you need a lesson in how much of a delightful humongous DORK Brendon Urie is, I recommend this: