A few more then :
Skiing : The act of using your jetting manoeuvers and using the landscape slopes to 'slide' along and effectively gain speed and momentum; it's a required skill to reach high speeds without OverDrive or to actually initiate OverDrive.
Gliding : Can be interpreted as Skiing (see above) but also can be denoted as the act of using your jets to remain airborne while moving. Used in conjuction with Skiing, it can lead to immense leaps from one side to the other of the battlefield.
LOL : Stands for "Laughing Out Loud" .... though most of the time, someone just finds an happening or comment funny
CTF : Capture the flag... one of the most popular team oriented gameplay in FPSes. Goal is to capture the flag of the other team and bring it back to your own base while your flag is present. Reaching the number of capture or having the highest number of captures after the time limit expires will net you victory.
TDM : Team Deathmatch; players are split into two teams and the team with the highest kill count at the end of the time or the team that reaches the set kill limit wins.
DM : Deathmatch; same as aforementionned TDM except that it is every man for himself.
Floater : Can refer to a player in midair, either of their own accord or from an attack that launched them.
Flag Pass : Throwing the flag to an ally to carry; quite a viable tactic both for giving the flag to someone with more health or throwing off enemies with an unexpected tactic.
Launcher : Not to confound with some weapons' names, a "Launcher" is any kind of projectile or appartus that will cause the subject to be moved from impact or use. In such, a Rocket Launcher is a 'Launcher' weapon since ground bound rockets can push an enemy away from the explosion.
Base Raping : Simply put, having one team assault and hold the enemy's base in hope to score easy kills by killing respawning enemies. (See Spawn Camper)
Camper : A player who doesn't move from his spot for a long period of time. There are active and passive campers; the active kind will camp an area and try to shoot out incoming enemies without regard of giving out their general position. Passive campers hold a position and attack any enemy that comes within their threat range; they also tend to remain concealed as to surprise the enemy. Passive Campers are not often frowned upon as they are most of the time people who assume defensive positions around the base. Likewise, some Active Campers might be the snipers; they'll hold a position in middle field and try to take down as many attackers as possible.
SC : Spawn Camper; refers to a player who remains close to a possible player spawning point in hope of getting easy kills on unexpecting targets.
Attacker : Player whom team or self appointed role is to secure objectives and engage the enemy head on.
Defender : Player whom team or self appointed role is to prevent the enemy from securing the objective.
Runner : Player whom team or self appointed role is to rush for objective while mostly disregarding the use of weaponry; Runners are usually specialists of Skiing and OverDriving.
Lamer : Player who uses tactics or features that are generally frowned upon. These tactics usually range from being rather effortless and yet effective and can lead to cause grief. Also used as an insult.
Grieffer : Player whom gives grief to another player or team in an attempt of being disruptive. Usually leads to the player being reported then banned.
-- Important Notions --
Noob/Noobie : A player who acts in irrationnal ways and purposefully ignore any sort of tactics and advice. Noobs are usually disruptive in their simple ways. Also a known insult of lesser degree that is scoffed off most of the time.
Newb/Newbie : Similar to a Noob/Noobie except that the player is not disruptive and actually trying to learn to play the game. Sadly, some experienced players tend to have little patience in showing the new players the ropes, leading frustrated newbies to regress into noobies.
Pro : Player who is recognised to be skilled in a particular way with the game. Some Pro focus on a single aspect to perfect while others attemp to learn and perfect all aspects.
Sore Loser : A player who blames his team's defeat on the other players of the team. Rarely is this case true and even so, it is considered to be a Sore Loser to put the blame on teammates.
Sore Winner : A player who takes victory to a whole new insulting level. Tends to flaunt openly and insult anyone he or she has had victory over. Does not tend to make friends easily... though Sore Winners are known to flock together at times (See "Clan").
Clan : A group of like minded players who decide to be recognised as a single entity. Clans can be beneficial and detrimental; they can lead to enjoyable sportsmanship in Clan vs Clan matches. They can, however, be detrimental if the clan is considered to be of a competitive level and constantly taking on public matches against regular players and imbalance the game's skill ratio.
Also, for who wonders, the mention of clan in "Sore Winners" was just a blatant joke
... Though some Clans can be Sore Winners as well as Sore Losers.Admin : The people you don't want to piss off

They're also helpful and responsible for the well being of Instant Action and their games. They are a resource of information and support when required. Admins are generally well intentioned and fair; those who aren't usually don't last long.