Damage dealers are referred to as "DPS" here. Calling them "DD's" will get you odd stares.

"HoT" : Never hear it in ARR, but that means "Heal over Time". These are spells like Regen that don't heal you a bunch right away, they heal you a little bit over time.

Characters are referred to as "characters". Rarely "avatars". Never use "toon" in these here woods.

"Main" is the character you play most of the time. They are the best equipped (usually). "Alts" are alternate characters you create to try something new. These characters may not be as well off in terms of gear.

"Gear" refers to equipment. Saying "I'm geared for coil." means you have equipment that is suitable for that task.

That's all I can think of.