Landing a Headshot (or critical hit) deals critical damage, and is done by attacking an opponent's head hitbox with a hero's weapon or ability. All headshots gain a natural 2.0x damage modifier, making them deal double the damage of a bodyshot (for example, McCree's Peacekeeper deals 70 damage to the body, but 140 damage to the head).
Some heroes can gain a natural 2.5x damage modifier when dealing a headshot in specific situations: Widowmaker, while firing Widow's Kiss in Sniper mode; Genji, while Deflecting scoped shots fired from Widow's Kiss; and Echo, while firing scoped shots as a Duplicated Widowmaker.
Not all attacks are capable of dealing critical damage, even when aiming at the head (some heroes are incapable of dealing critical damage entirely). Note that in game modes that restrict all damage to headshots only, some heroes and deployables may not deal any damage whatsoever (like Symmetra, Moira, turrets, and B.O.B.), while others may become immune to damage (like Hammond, Bastion, turrets and barriers).
Example Video showing Hitboxes 
Weapons and Abilities capable of dealing Headshots
- Ashe: The Viper.
- Bastion: Configuration: Recon.
- Baptiste: Biotic Launcher primary fire only.
- D.Va: Fusion Cannons and Light Gun.
- Doomfist: Hand Cannon.
- Echo: Tri-Shot.
- Landing a headshot while Duplicating another hero may deal critical damage.
- Genji: Shuriken and Fan of Blades.
- Landing a headshot while Deflecting may deal critical damage.
- Hanzo: Storm Bow, Storm Arrows and Sonic Arrow.
- The physical arrow loosed while using Dragonstrike may deal a headshot.
- Lúcio: Sonic Amplifier.
- McCree: Peacekeeper primary fire only.
- Despite the official description, Deadeye always targets the body and may not deal headshots.
- Mei: Endothermic Blaster secondary fire only.
- Mercy: Caduceus Blaster.
- Orisa: Fusion Driver.
- Reaper: Hellfire Shotguns.
- Roadhog: Scrap Gun and Whole Hog.
- Soldier: 76: Heavy Pulse Rifle primary fire only.
- Tactical Visor always targets the body and may not deal headshots.
- Sombra: Machine Pistol.
- Torbjörn: Rivet Gun.
- Tracer: Pulse Pistols.
- Widowmaker: Widow's Kiss Assault and Sniper modes.
- Headshots dealt in Sniper mode have a natural 2.5x damage modifier, instead of the usual 2x modifier.
- Wrecking Ball: Quad Cannons.
- Zenyatta: Orb of Destruction and Orb Volley.