Conditions and Boons

Condition effects are negative effects placed on opponents. Better known as debuffs.
Boon effects are positive effects placed on allies. Better known as buffs.
Control effects are effects that temporarily restrict a foe's ability to move and/or use skills.
Other effects are effects that don't fall into the three categories above.

Conditions
Bleeding - Inflicts X damage per second. Stacks in intensity.
Blind - Causes the target's next hit to miss instead.
Burning - Inflicts X damage per second. Stacks in duration.
Chilled - 66% movement-speed reduction & 66% slower skill recharge while active. Stacks in duration.
Confusion - Inflicts X damage each time a foe uses skill. Stacks in intensity.
Crippled - Target moves 50% slower. Stacks in duration.
Fear - Causes the target to run directly away from the caster.
Immobilized - Immobilizes and prevents dodge. Stacks in duration.
Poison - Inflicts X damage per second. Reduces outgoing heals by 33%. Stacks in duration.
Vulnerability - X armor reduction. Stacks in intensity.
Weakness - 50% of attacks are glancing and endurance regeneration is reduced by 50%. Stack in duration.

Boons
Aegis - Block the next attack. Exclusive to guardians.
Fury - 20% Critical Chance increase. Stacks in duration.
Might - X damage per attack increase. Stacks intensity.
Protection - 33% damage reduction. Stacks in duration.
Regeneration - Regenerates X health per second. Stacks in duration.
Retaliation - Does X damage to an opponent each time they hit you. Stacks in duration.
Swiftness - 33% movement speed increase. Stacks in duration.
Vigor - Faster endurance regeneration. Stacks in duration.

Control
Cripple, Chilled, and Immobilize - slow and prevent movement.
Sink and Float- cause a creature to rise or fall in water.
Pull - forces an enemy to your location.
Wards - limit enemy movement beyond a line or circle.
Blind, Daze, and Weakness - reduce the damage enemies can deal.
Confusion and Retaliation - is a condition which deals damage each time a foe attacks.
Shield of Absorption or Whirling Defense - can block or deflect attacks.
Fear - causes enemies to run from you and prevents skill usage.
Knockback and Launch - force an enemy away from you and interrupt skills.
Knockdown and Stun - prevent any movement or skill usage.

Other
Distortion - is an effect exclusive to mesmers which allows them to evade attacks for some time.
Frenzy - is an effect that increases damage done with critical hits.
Frosty Armor - is an effect which protects against incoming attacks.
Haste - is an effect which improves endurance regeneration.
Quickening Zephyr - is an effect which increases movement speed.
Quickness - is an effect that makes all skills and actions twice as fast
Rejuvenation - is an effect that regenerates health. This effect is exclusive to the guardian profession through the Virtue of Resolve.
Smiting - is an effect that causes every fifth attack to apply burning. This effect is exclusive to the guardian profession through the Virtue of Justice.
Stability - is an effect that makes the target immune to crippled, chilled, knockdown, launch and knockback.
Stealth - is an effect which allows the user to be invisible to enemy players and avoid being spotted by enemy NPCs. A stealthed character will appear as a transparent outline to allies. In PvE, a foe will stop attacking if the target becomes stealthed. When thieves enter stealth, their first weapon skill changes to a special stealth skill.
Invulnerability - is an effect that prevents the target from taking damage or receiving conditions. It is automatically placed on many boss monsters during cutscenes. It is also placed on rallied players for one second.
Power of the Mists - is an effect that applies in World versus World and Player versus Environment and provides bonuses to the player depending on the war score of their home server. War scores are tiered with each tier unlocking an additional or upgraded bonus.