As Dark Souls II is an Action RPG with duel-type combat, you may find terminologies common in RPGs and fighting games used by the community.
Aggro - Possibly short for "Aggravate", it is the action of provoking an enemy or NPC into attempting to engage combat with the player. While this usually means walking within a certain distance of a monster's line of sight, it can also refer to pulling a monster from afar, or attacking certain non-hostile NPCs (which turns them hostile, and provokes them into fighting you).
AoE - Area of effect; refers to attacks that only affect those within a specifc target range or distance
AR - Attack Rating is the total rate of attack (not damage) added from a weapons Physical/Magic/Fire/Lightning/Dark damage. This number is listed in the Weapon Attribute Screen. This number applied against an enemy's Physical/Magic/Fire/Lightning/Dark defense is what gives the actual damage.
Bonfire Intensity - In Dark Souls II, Bonfire Intensity affects the difficulty level of the game. Also, many new enemies and item drops appear only at Bonfire Intensity 2 or above.
Every bonfire starts at Bonfire Intensity 1. Burning a Bonfire Ascetic increases the Intensity of that bonfire by 1, while every New Game Plus cycle adds 1 to the Intensity of every bonfire. With Ascetic use, you can see almost all the effects of NG+ in NG. See Bonfire Ascetic and New Game Plus for more information.
BS - Stands for "Backstab"
Caster - Descriptor for a player who relies on spells for combat, typically has sacrificed HP and defense for high Intelligence and Attunement. Does well in ranged combat but can do well in close quarters given the proper spells.
Casul - Word (misspelling being intentional) used to describe a player who uses cheeky methods, such as a ganker, or someone who is very easy to take down. This was used in the first game to describe people who fell victim to the legend know as "Giantdad." This is most commonly used when the invader or player in question has successfully taken down either a single person or a group, usually as an insult.
Cheese - The act of half-attempting a boss or enemy through easier and unofficial means.
Farming - Exploring a particular area to hopefully grab many rare items from enemies or collect souls. Many players will repetitively travel throughout certain areas of the game in hopes of gathering a rare item from a particular enemy such as Human Effigies.
Ganker - Players who brought allies to a PvP situation, hoping to win by number advantage. In Dark Souls, this terminology usually applies to host players who summon phantom(s) and deliberately waits for an invasion to happen so that the invader usually ends up in a 1v2 or 1v3 situation.
Grinding - Doing repeated activity (such as killing enemies) to gain experience points in order to level up faster. This basically means "soul farming" as souls is used for levelling up. In Dark Souls 2, because the enemies stop spawning after being killed a number of times, players will have to use Bonfire Ascetics to respawn them.
Jumping - When being invaded/invading, you attack the invader/are attacked by the host immediately after spawning (i.e. "getting a jump on the invader/host").
NG+ - Stands for New Game Plus
Praise the Sun! - A common message seen throughout to indicate approval or loyalty of an object, view, or covenant. It is just a mere phrase that is put to show happiness or just because the player felt like putting something down.
Pulling - A method used to draw out an enemy for tactical purposes, like separating an enemy from a group, or luring them into a more preferable terrain. This is commonly done with bows, but can be done with a variety of long-ranged attacks, including the use of spells and consumables.
PvE - Stands for Player vs Environment.
PvP - Stands for Player vs Player.
Roll BS - Stands for "Roll backstab". The act of dodging an attack by rolling behind, then immediately backstabbing the opponent.
SL - Stands for "Soul Level"
SM - Stands for "Soul Memory"
Spam - A repeated action (usually an attack) done generally to dish out large amounts of damage in disregard to one's safety. (e.g. spamming Great Combustion)
SS - Stands for "Soul/Summon Sign"
Tank - (n.) - Used to describe a player in heavy armor and shield. Tends to have high HP, Defense and Poise. Unsubtle, but definitely not inconsiderable in combat.
Tank through - (v.) - The act of achieving victory by means of being a "tank" rather than by dodging, parrying, ect.
Whiff - Refers to an attack that fails to connect with the opponent. Whether it misses them completely, or actually hits a character, but because that character was in the middle of a dodge-roll at the time, the attack does no damage. Whiffing an attack usually makes your character open to counter attacks. In Dark Souls, some weapons have special whiff animations that makes the player slower to recover when missing an attack, making whiffing more dangerous (e.g. most axes, halberds, greataxes)