Teching
Teching, also known as "down recovery", lets you avoid being hit OTG by pressing 
+Attack Buttons as you are knocked down. Your character will recover in the air as opposed to staying on the ground. This is important, because otherwise opponents can hit you with fake OTG infinites.
Instant Kill
Instant Kill moves are initiated by inputting P+K, and then completed by inputting 
+Attack Button during the "Sakkai" time - the time during which the screen turns red and game is paused. Successfully landing an IK wins you the current round and all remaining rounds in the game. An opponent's instant kill can be negated by inputting 
+Attack Button during the Sakkai time.
Instant Kills can also be triggered by performing an Instant Block.
Guard Cancel
Guard Cancels, known officially as "Dead Angle Attacks", are special moves performed during blockstun. Any special move can be used as a guard cancel for free. This allows you to interrupt your opponent's offense reversal style specials.
The community does not often use the official name in order to avoid confusion with more recent and popular titles where Dead Angle Attacks behave entirely differently.
GCJ Glitch
The Guard Cancel Jump, or GCJ, is a a niche technique that allows you to jump during blockstun. To perform a GCJ, input jump immediately after blocking an attack. If performed correctly, your character will levitate into the air while in blockstun without going through their prejump animation. Blockstun sustained before leaving the ground persists as you become airborne.
This phenomenon is still being researched, and is not fully understood at this time.
Charge Attacks
Each character, except for 
Axl, 
Testament, and 
Justice, has a specific special move that can be charged. To charge a move, perform that moves input, but use the Respect button instead of the normal attack button. Your character will play a unique animation and gain a charge level. That move will become powered up for each level that you charge it.
Charge Canceling
Charge Canceling is the mechanic which allows most characters to perform infinite combos.
Guard
Guarding, known commonly as blocking in the FGC, works as expected in GGML. Most moves can be blocked low, high, or both, with a few special moves being unblockable. Blocked normal moves will usually deal no damage to the blocker, and specials will do significantly reduced damage, known as chip damage Damage incurred when blocking an attack. Typically, only Special Attacks and Super Attacks have chip damage, but there are exceptions..
Faultless Defense
Faultless defense is performed by holding the Respect button while blocking an attack. This will negate chip damage received from that attack at the cost of a moderate amount of meter, known in game as the Chaos Gauge.
Instant Block
Instant Blocking is performed by inputting block at the same moment that an opponent's attack would hit you. Doing so will cause you to perform an Instant Kill Sakkai on the opponent immediately.
Dust Attack
Dust Attacks are overhead attacks which are performed by inputting S+H. If you hit the opponent with a dust attack, the opponent will be sent flying in a special launch animation for unique combo opportunities. Follow the opponent into the air by inputting .
Clashes
A clash occurs when 2 attacks connect with each other. When a clash happens, both players can cancel their current move into another attack.