A skald is, in a nutshell, a Viking bard. In the real world, it was the title of a class of poet-composers popular in the halls of Norse cultures, mostly Scandinavian and Icelandic, who made their living by reciting epic sagas relating to heroes both historical and mythical, as there wasn't much of a difference between the two at the time. Mostly they were given money by the Norse leaders to compose poems to honor and commemorate the great deeds of those leaders.
Naturally, Advanced Dungeons & Dragons tried to provide this as an option, once Bards stopped being an insane example of multiclassing in their own right. The skald first appeared as one of the bardic kits in The Complete Bards Handbook, where it was basically a more combat-capable bard, with increased weapon proficiency and a special "War Chant" buffing ability, although they're less good at the more thief-like abilities of the bard. The result is similar to, but different from, the Blade kit, which is more of a bardic swashbuckler.
Subsequent editions moved away from directly referencing the idea, but it survives in the /tg/ lexicon as a shorthand name for the "Bard-barian" concept; a multiclassed Bard/Barbarian who can blend the bard's support role with greater melee combat ability. The blend isn't necessarily the most harmonious, but that's more an issue with multiclassing as a whole.
Pathfinder ran with the concept, making the skald a bard/barbarian mix in the Advanced Class Guide, which featured similar blent classes. It gets martial weapon/medium armor proficiencies, like a barbarian, but its signature power is a "rage song," which puts all listeners into barbarian rage. They even get a suite of rage powers they can apply to the song in the process. As variant Bards, Skald are still tier 3. Raging Song is less useful out of combat than Inspire is, but gaining Spell Kenning to pull utility spells from their ass makes up for that.
4th edition brought Skalds back as a whole new subclass for bards through their "Heroes of the Feywild" sourcebook. Hybrid Arcane/Martial Leaders, they have the ability to substitute Charisma for Strength when making one-handed melee weapon attacks, gain some bonus Daily Bard powers, and have the Skald's Aura class power, which lets allies spend healing surges as a minor action 1/encounter. Plus, with how hybrid-classing worked, the "Bardbarian" was probably easier to pull off here than in 3rd edition.
D&D 5e has a College of Valor bard subclass, which functions roughly along the lines of the Skald kit, being a more frontline fighter variant of the bard, in comparison to the magic-focused College of Lore bard or the swashbuckling College of Swords bard.
|Dungeons & Dragons 4th Edition Classes|
|Player's Handbook 1:||Cleric - Fighter - Paladin - Ranger - Rogue - Warlock - Warlord - Wizard|
|Player's Handbook 2:||Avenger - Barbarian - Bard - Druid - Invoker - Shaman - Sorcerer - Warden|
|Player's Handbook 3:||Ardent - Battlemind - Monk - Psion - Runepriest - Seeker|
|Heroes of X:|| Blackguard - Binder - Cavalier - Elementalist - Hexblade - Hunter|
Mage - Knight - Protector - Scout - Sentinel - Skald - Slayer - Sha'ir - Thief
Vampire - Warpriest - Witch
|Settings Book:||Artificer - Bladesinger - Swordmage|
|Others:||Paragon Path - Epic Destiny|
|Dungeons & Dragons 5th Edition Classes|
| Barbarian - Bard - Cleric - Druid - Fighter - Monk |
Paladin - Ranger - Rogue - Sorcerer - Warlock - Wizard
|Artificer - Mystic|
|The Classes of Pathfinder|
|Core Classes:|| Barbarian - Bard - Cleric - Druid - Fighter - Monk |
Paladin - Ranger - Rogue - Sorcerer - Wizard
| Alchemist - Antipaladin - Cavalier |
Inquisitor - Oracle - Summoner - Witch
| Arcanist - Bloodrager - Brawler - Hunter - Investigator |
Shaman - Skald - Slayer - Swashbuckler - Warpriest
| Kineticist - Medium - Mesmerist |
Occultist - Psychic - Spiritualist
|Ultimate X:||Gunslinger - Magus - Ninja - Samurai - Shifter - Vigilante|