What's a Keyword?
Because Path of Exile is a very deep game with a lot of mechanics, the game needs some short-hand to express what things affect what other things.
In other words, instead of saying "10% Increased Damage For Effects That Shoot A Mobile Thing That Directly Strikes A Target", an item can just say "10% Increased Projectile Damage". Then all you need to do to check whether that damage is compatible with certain skills is to check whether those skills count as Projectiles. The closest analogy I can think of for most players are the keywords used in collectable card games like Magic: the Gathering or Hearthstone.
You don't need to know all these keywords by heart when you start, but they're useful for understanding what things work with your build and what things don't.
- Ailment: Poison, Bleed, Ignite, Shock, Chill, or Freeze. See the main "Ailments" page for more details.
- Ally: Party members, along with the Totems and Minions summoned by you or party members. Strictly speaking, You are not an Ally, although most mechanics that affect Allies affect you as well.
- Area, AoE: Damage dealt, or effects applied, to targets within a certain distance of a point or within a particularly shaped region. Cone-shaped effects are generally Area effects, as are circle-shaped ones. Note that damage done indirectly by effects applied in an Area usually is not considered an Area effect in its own right. So, for example, a Damage over Time effect applied to all targets in an area usually does not count as "Area Damage" (because the application of the effect is done in an area, but the damage is not - it's done by the effect, which is already on the target before any damage is done). However, modifiers to Area of Effect (as opposed to Area Damage) usually do affect such skills.
- Attack: A specific type of Skill. Attacks in some way involve your weapon's damage. Attack can also be a type of Damage Source, when the damage is dealt by an Attack Skill (although not all damage dealt by Attacks has Attack as its damage source: most notably, Damage Over Time effects do not deal Attack damage, they deal Damage over Time).
- Aura: An effect applied to units around you. Most auras affect you and Allies and provide beneficial effects, although harmful Auras do exist.
- Burning: A Fire Damage over Time effect. Not the same thing as an Ignite: all Ignites are Burns (because they do deal Fire Damage over Time), but not all Burns are Ignites. Don't confuse the two!
- Curse: A particular category of debuff applied by Curse skills. By default, an enemy can only have one Curse applied to it, although various effects can increase this limit.
- Damage over Time: One possible Damage Source. In particular, a Damage over Time effect is not an Attack or a Spell, and won't be affected by modifiers to Attacks or Spells unless specified otherwise.
- Hit: An impact of a damage source that could do damage (even if that damage is somehow prevented). Applications of negative effects, and the damage of Damage over Time effects, do not count as Hits. Many other effects, like Critical Strikes, Resistance Penetration, and conditional damage effects, only apply to Hits (and not to Damage over Time).
- Mine: A type of Skill that places Mines for you. Mines use your skills when detonated, and therefore benefit from your stats in the damage they deal. For more on these, see "Traps, Totems, & Mines".
- Minion: A unit summoned by a Minion skill. Minions count as Allies. In particular, Totems are not Minions. Minions' Attacks and Spells use the Minion's own stats, and do not benefit from yours unless they specifically state that they affect Minions. All damage dealt by Minions is affected by Minion Damage modifiers.
- Nearby: This is actually not a Keyword, even though it may look like one given that, say, Recently is one. Nearby just means the effect is range-limited, but different effects can have different ranges even if both say Nearby.
- On Full Life/Mana/Etc.: At 100% of your maximum Life/Mana/Etc. If any of your Life/Mana/Etc. is Reserved, you can never be On Full Life/Mana/Etc.
- On Low Life/Mana/Etc: Below 35% of your maximum Life/Mana/Etc. If more than 65% of your Life/Mana/Etc. is Reserved, you will always be On Low Life/Mana/Etc.
- Projectile: Damage dealt, or effects applied, by a mobile projectile that strikes targets as it passes. Effects that apply in an area, even a moving one, are usually not Projectiles. Note that damage done indirectly by effects applied by a Projectile usually is not considered an Projectile effect in its own right. So, for example, a Damage over Time effect applied by a Projectile usually does not count as "Projectile Damage" (because the application of the effect is done by a Projectile, but the damage is not - it's done by the effect, which is already on the target before any damage is done).
- Recently: In the last 4 seconds.
- Skill: A general term for all abilities you can use. Attacks, Spells, Warcrys, and others are types of Skill.
- Spell: A specific type of Skill. Spells do not use your weapon's damage (although they may benefit from stats on your weapon). Spell can also be a type of Damage Source, when the damage is done by a Spell Skill (although not all damage dealt by Spells has Spell as its damage source: most notably, Damage Over Time effects do not deal Spell damage, they deal Damage over Time).
- Totem: A unit you summon that casts your Spells or performs your Attacks for you. Totems use your skills, and therefore benefit from your stats in the damage they deal, although they have their own health, resistances, armour, and so on. For more on these, see "Traps, Totems, & Mines".
- Trap: A type of Skill that places Traps for you. Traps use your skills, and therefore benefit from your stats in the damage they deal. For more on these, see "Traps, Totems, & Mines".