Khorne Berzerkers
Khorne Berzerkers (languagefaggotry note: that should be Khornate Berzerkers, but "Khorne Berzerkers" is the official title) are Space Marines dedicated to Khorne, the god of blood, war, and fucking your shit up.
Most Khorne Berzerkers come from the World Eaters Traitor Legion. Their Primarch, Angron, decided that his most elite Marines should have partial lobotomies, just like himself. When this was done, the only emotion they could feel was RRRAAAAGGGGEEE. As the Horus Heresy dragged on, the World Eaters would eventually turn to the worship of Khorne, with the Berzerkers leading the way. Of course, they lost, fled to the Eye of Terror, and have spent the past 10,000 years worshipping Khorne the only way they know how: BATTLE, GLORIOUS BATTLE! Some renegades have joined them, but very few Space Marines are angry enough to keep up. They usually hang out in small squads to company to chapter size groups although the bigger formations are always temporary; there is too much RAGE going on in their minds to work together for long, because, after all, they are as likely to spill their comrades blood as an enemy's.
On the Table Top
In game terms, they're basically regular Chaos Space Marines with Chainaxes and bolt pistols (which they hardly use but make good clubbing instruments). They also get a higher Weapon Skill, Fearless and Furious Charge, as well as Marks of Khorne, which gives them Rage and Counter-Attack. This means that they absolutely butcher in melee... if they can get there. With a hefty price tag of 19 points per model, poor transport options (expensive Land Raider or assault-ramp-less Rhino), and the overall nerfing of melee in 7th Edition (and the sheer fragility of one wound MEQs in the current meta), it's very difficult to get them there.
That's not to mention that regular Chaos Marines can be kitted out for close combat almost as well as the Berzerkers, and they are both cheaper and scoring with Objective Secured. Not at all true, because the most Zerky CSM squad will be 20p cheaper, but not Fearless or at WS5. So it's always more worth it to take regular Zerks, which are scoring now as well. Remember, my angry friends, that the 7th Edition made everything scoring and changed Counter-Attack so it no longer requires a Ld test. A MoK Lord and this fun guy make them Troops.
In Warhammer Fantasy Age of Sigmar:
Surprisingly, Khorne has never had dedicated worshipper-troops in Warhammer Fantasy, just Chaos Warriors/Knights sporting his Mark, though they certainly fulfill the role fluff-wise. The End Times adds two units even closer to the spirit, via the Archaon book (leaked early in White Dwarf). Both have the basic fluff of being particularly advanced and crazed Khorne worshippers who have started to mutate into even more deadly fighters, complete with growing muscles so big they're starting to burst out of their Chaos Armor.
They're both Infantry sporting a Movement 4, Weapon Skill 6, BS 3, Strength 4 (Strength 5, for Wrathmongers), Toughness 4, Wounds 3, Initiative 5, Attacks 3, Leadership 8 profile. The differences are subtle.
Skullreapers are a 40pts per model Special choice who run around sporting paired weapons (+1 Attack), which they can upgrade to Paired Ensorcelled Weapons (+1 Strength, attacks count as magic, +1 Attack) for +5 points per model. Since the minimum size of a squad is five, even without taking a Champion (who gets another +1 Attack), a bare-bones Skullreaper squad with Ensorcelled Weapons is costing you 225 points for something that can dish out 20 S5 hits that will actually hit damn Ethereal troops at Weapon Skill 6. Not too bad at chewing up basic mooks, all things considering.
Wrathmongers are even madder than Skullreapers; they cost 55 points each and use up a Rare slot, thanks to their sporting Paired Wrath-flails (+1 Strength in first round of combat - yes, that means they're S6 during the first round, extra attack, Impact Hits D3). Again, bare minimum squad is 5 for 275 points, which is dishing out 5D3 S5 Impact hits on the charge and then 15 S6 hits directly afterwards... pretty damn choppy.
The later Age of Sigmar setting created an entirely new subset of Khornate-exclusive warbands known as the Bloodbound, who are essentially Khornate Warriors of Chaos in more thematically unified armour, with their beard fetishes enhanced. There are the Blood Warriors, who feel such unmatched fury that it becomes a physically tangible force that emanates from their bodies like a heatwave. They are apparently so manly that they can continue to hack their enemies into tiny bits even while they're in their death throes. Slaughterpriests, who are basically Khornate Evangelical priests. Bloodreavers, basically you're bog-standard Chaos Marauders with shitty looking beards. Bloodstokers, fat lion-tamers who drive their fellow warriors to untold heights of rage by being insufferable pillocks. Bloodsecrators, what's the fucking point?
Non Warhammer Berserkers
The term "Berserker" comes from the Viking "Baer-sark", or "bear-shirt", referring to dreaded warriors who would dress themselves up in bearskins and likely worked as champions for a lords or jarls, fighting duels in their stead. Some even believed they could transform into wolves and bears, making them a possible origin for the myth of the werewolf. Similarly frenzied warriors appeared in many European Bronze and Iron Age cultures, like the Celts and Picts, they however were not called berserkers, and even among vikings, people who went "berserk" weren't called berserkers in any of the sagas remaining to this day. However a 13th century norse translation of a french story: "Yvain, Le Chevalier au Lion", translates the french word for champion into the norse word for berserker.
The Barbarian roleplaying game class, particularly in Dungeons & Dragons and Pathfinder is based significantly on the concept of the common misconception of a Berserker, as well as on Conan (who himself by D&D lore is more of a Fighter with thief skills). Both games also have a Berserker-themed archetype for fighters; the Berserker in Pathfinder, and the Battlerager in 4th edition.
Berserker is also the name of a ridiculous song performed by Love Among Freaks that appeared in the movie Clerks in 1994.