Here's a quick non-perfect list of terms you may seen thrown around:
IC:
Inexperience-Challenged player (that's me!)
SE:
Semi-experienced challenged player. These are players with a good amount of experience on the site.
Scum:
General term for non-town alignments, in the case of this game, the mafia players
L-#:
The number of votes before a player is lynched. (Example, "Aeronaut is at L-4" means I am 4 votes away from being lynched)
LYLO:
Lynch (correctly) or lose. This means that if a lynch on town occurs, the mafia immediately wins because they'll have at least 50% of the numbers.
BP:
Bulletproof, a role that can get shot by the mafia once and not die.
Doc:
Doctor, a role that can attempt to protect people from nightkills.
JK:
Jailkeeper, a role that both protects and blocks its target.
Scummy:
An adjective for any action (or player) more likely to come from or be mafia than town.
Meta:
A player's past style of playing from completed games as a given role as compared to their current play.
PR:
Power Role, any role that is not vanilla town or mafia goon.
OMGUS:
(Oh my god you suck) is where a player essentially votes a player for voting them first. Considered not good play.
NK:
Night Kill. The mafia's kill they get to use once a night.
PT:
Private Topic: The hidden secret forum where the mafia may talk at night and pregame.
Crumb:
A hidden message that can be revealed as needed or will be found after your death hinting on what you do with a power role, or that you are that role.
Claim:
An outright claim of your role. Please note you may NOT post your role PM, you can only paraphrase it. DO NOT DO THIS UNTIL IT IS COMPLETELY NECESSARY.
CC:
Counter Claim. This is when someone claims a role that makes a previous role claim false, exposing the previous player as lying.
Hammer:
The vote that brings somebody from L-1 to lynched. IT'S IMPORTANT TO DECLARE INTENT TO HAMMER BEFORE DOING IT!
Mislynch:
When town mistakenly lynches a player who was town.
Let me know if I missed anything!
Also, here are some links that may also help you out.
Remember, the Wiki is your friend!