Act I Hirelings: Rogue Scouts
The Hirelings of Act I are available from Kashya in the Rogue Encampment. As Rogues, they specialize in the use of Bows and can be useful for ranged support.

Act 1 Rogue Scouts can now use Crossbows as well as Bows.

Fire: Fire Arrow, Exploding Arrow, Critical Strike, Inner Sight, Pierce, Immolation Arrow (NM/Hell)
Cold: Cold Arrow, Ice Arrow, Critical Strike, Inner Sight,Pierce, Freezing Arrow (NM/Hell)

Act II Hirelings: Desert Mercenaries
The Hirelings of Act II are available from Greiz in Lut Gholein. They specialize in Melee Attacks.

Act 2 Desert Mercenaries can now use Shields and Javelins (note they do not benefit from block on shields)

Recovery: Jab, Vengeance, Conversion (NM/Hell) Prayer Aura
Offensive: Jab, Vengeance, Conversion (NM/Hell) Might Aura
Defensive: Jab, Vengeance, Conversion (NM/Hell) Thorns Aura

Act III Hirelings: Ironwolves
The Hirelings of Act III are available from Asheara in Kurast. They specialize in Lightning, Magic, Fire, and Cold spell attacks.
They also now have Holy Auras that pierce enemy resistances.

Act 3 Iron Wolves can now use Staves, and Wands.(They do not benefit from block on shields)

Lightning: Nova, Charged Bolt, Lightning, Lightning Mastery,Holy Shock Aura Chain Lightning (NM/Hell)
Cold: Glacial Spike, Ice Bolt, Cold Mastery, Shiver Armor,Holy Freeze Aura, Blizzard (NM/Hell)
Fire: Firestorm, Firebolt, Fireball, Holy Fire Aura, Fire Mastery, Meteor (NM/Hell)
Poison & Bone: Teeth, Bone Spear, Bone Armor, Poison Resistance Pierce Aura, Bone Spirit (NM/Hell)

Act V Hirelings: Barbarians
The Hirelings of Act V are available from Qual-Kehk in Harrogath once you have finished his quest.

Act 5 Barbarians can now use Axes and Shields (they do not benefit from the block on shields)

Concentrate, Zeal, Stun, War Cry (NM/Hell)
Werewolf: Zeal, Feral Rage, War Cry(NM/Hell)
Werebear: Maul, Shockwave, War Cry(NM/Hell)

In HU, you can now fully equip a mercenary with Gloves, Boots, Belt, Rings, and an Amulet along with the regular Helm, Body Armor and Weapon. To account for this they no longer gain resistances upon leveling.

Mercenaries deal 1/4th damage to bosses and take 10x damage from bosses.

Mercenaries do not get experience from party member kills, but they do from your summoned monsters/characters.

Hirelings will not gain experience once they have reached your level. They will begin to gain experience again once you level up.

Credits to Arreat Summit and Steel