As Dark Souls 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
BP - Stands for "Black Phantom". A borrowed phrase from Demon's Souls referring to the hostile phantom form of an NPC or the phantom form of invading players. The Dark Souls equivalent of the Demon's Souls' Black Phantom is "Dark Spirit".
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.
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 Humanities.
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.
Giant MoM - Sometimes a derogatory term, it refers to players who use the combination of the Giant Set with the Mask of the Mother. This armor combination is considered to endow the most overall survivability in the game, while also giving a very desirable amount of poise. The immense mathematical incentive to wear this armor has been met with controversy, sometimes being blamed for inhibiting aesthetic creativity and diversity.
Giant Dad - Similar to "Giant MoM"; an often-derogatory term denoting players who combine the Giant Set with the Mask of the Father. This armor set provides marginally less survivability than its Mask of the Mother counterpart, but raises the player's equip burden, allowing for easier fast-rolls and Dark Wood Grain Ring use while wearing an extremely protective suit of armor. Much like its counterpart, this armor set is often criticized for reducing variation in equipment and playstyle due to its statistical excellence.
Griefer - Players who take pleasure in bringing grief to other player, even if the griefer does not gain significant gameplay reward or even at a disadvantage. In Dark Souls, it is usually a player that uses high level gear on a low level character (so that it matches the online level range) to invade people. They mostly invade people on Undead Burg and other starting areas to easily win fights against novice players.
Grinding - Doing repeated activity (such as killing enemies) to gain experience points in order to level up faster. In Dark Souls, this basically means "soul farming" as souls is used for levelling up.
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
Oki - Short for "Okizeme", which is the Japanese term for the tactical situation that is present against an enemy that is rising up off the ground, typically against someone who is recovering from being knocked down. Example of okizeme attacks (often themselves referred to as "okizemes" or "okis") in Dark Souls is swinging your heavy weapon (or casting a spell) while the opponent are getting up (and thus unable to do anything) and timed it so that the attack connects right after the opponent's invincibility wears off. Another example is the "chain backstab".
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.
PTD/PTDE- Stands for "Prepare To Die Edition", and refers to content that is available for the PS3 and Xbox 360 through the purchase of Artorias of the Abyss (DLC), and content on the PC that was not featured on the original release of Dark Souls.
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.
SL - Stands for "Soul Level"
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 - 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.
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)
ASS - Stands for the weapon Astora's Straight Sword.
Bcat - Stands for the catalyst Beatrice's Catalyst
BEO - Stands for the consumable Blue Eye Orb
BKGA - Stands for the weapon Black Knight Greataxe
BKGS - Stands for the weapon Black Knight Greatsword
BKH - Stands for the weapon Black Knight Halberd
BTR - Stands for the ring Blue Tearstone Ring
CGSR - Stands for the ring Covetous Gold Serpent Ring
CMW - Stands for the miracle Crystal Magic Weapon
CPR - Stands for the consumable Charcoal Pine Resin
CSSR - Stands for the ring Covetous Silver Serpent Ring
CREO - Stands for the consumable Cracked Red Eye Orb
CSS - Stands for the miracle Crystal Soul Spear
Ctal - Stands for the talisman Canvas Talisman
DBF - Stands for the weapon Dragon Bone Fist
Dcat - Stands for the catalyst Demon's Catalyst
DGA - Stands for the weapon Demon's Greataxe
DGH - Stands for the weapon Demon's Great Hammer
DGM - Stands for the weapon Demon Great Machete
DKGA - Stands for the weapon Dragon King Greataxe
DMT - Stands for the talisman Darkmoon Talisman
DSGB - Stands for the weapon Dragonslayer Greatbow
DSS - Stands for the weapon Dragonslayer Spear
DWG/DWGR - Stands for the ring Dark Wood Grain Ring, also known as the "Flip Ring"
EF - Stands for the miracle Emit Force
GCFB - Stands for the pyromancy Great Chaos Fireball
GCS - Stands for the shield Grass Crest Shield
GFB - Stands for the pyromancy Great Fireball
GHE - Stands for the miracle Great Heal Excerpt
GHSA - Stands for the sorcery Great Heavy Soul Arrow
GLS - Stands for the miracle Great Lightning Spear
GMB - Stands for the miracle Great Magic Barrier
GMW - Stands for the sorcery Great Magic Weapon
GPR - Stands for the consumable Gold Pine Resin
GSA - Stands for the sorcery Great Soul Arrow
HCSM - Stands for the sorcery Homing Crystal Soulmass
HSA - Stands for the sorcery Heavy Soul Arrow
HSM - Stands for the sorcery Homing Soulmass
HSW - Stands for the leggings Hollow Soldier Waistcloth
HW - Stands for the sorcery Hidden Weapon
Icat - Stands for the catalyst Izalith Catalyst
Ital - Stands for the talisman Ivory Talisman
KJ - Stands for the miracle Karmic Justice
Mcat - Stands for the catalyst Manus Catalyst
MoM/MotM - Stands for the headpiece Mask of the Mother
MLBH - Stands for the weapon Moonlight Butterfly Horn
MLGS - Stands for the weapon: Moonlight Greatsword
MSGS - Stands for the weapon: Man Serpent Greatsword
NLR - Stands for the area New Londo Ruins
Ocat - Stands for the catalyst Oolacile Catalyst
OIC - Stands for the catalyst Oolacile Ivory Catalyst
PW - Stands for the pyromancy Power Within
REO - Stands for the consumable Red Eye Orb
RoEE/RotEE - Stands for the ring Ring of the Evil Eye
RoF - Stands for the ring Ring of Fog
RoS - Stands for the ring Ring of Sacrifice
RoSF/RotSF - Stands for the ring Ring of the Sun's Firstborn
RoSP - Stands for the ring Ring of Steel Protection
RoFaP - Stands for the ring Ring of Favor and Protection
RotSP - Stands for the ring Ring of the Sun Princess
RPR - Stands for the consumable Rotten Pine Resin
RRoS - Stands for the ring Rare Ring of Sacrifice
RTR - Stands for the ring Red Tearstone Ring
Scat - Stands for the catalyst Sorcerer's Catalyst
SKSS - Stands for the weapon Silver Knight Straight Sword
SLB - Stands for the miracle Sunlight Blade
SLS - Stands for the miracle Sunlight Spear
SoA - Stands for the headpiece Symbol of Avarice
SSS - Stands for the weapon Sunlight Straight Sword
Stal - Stands for the talisman Sunlight Talisman
TBC - Stands for the catalyst Tin Banishment Catalyst
TCC - Stands for the catalyst: Tin Crystallization Catalyst
TDC - Stands for the catalyst Tin Darkmoon Catalyst
ToG - Stands for the area Tomb of the Giants
Ttal - Stands for the talisman Thorolund Talisman
TWoP - Stands for the miracle Tranquil Walk of Peace
VoD - Stands for the area Valley of Drakes
VR - Stands for the weapon Velka's Rapier
VT - Stands for the talisman Velka's Talisman
WDB - Stands for the sorcery White Dragon Breath
WoG/WotG - Stands for the miracle Wrath of the Gods
WP - Stands for the weapon Washing Pole