Many Warhammer 40K factions use squads of motorbike-mounted soldiers.
Bike Squads are fast attack Space Marine squads utilizing bikes for fast deadly tactics. The bikes themselves are armed with Twin Linked Bolters and the riders can carry the occasional special weapon, mostly meltaguns. Bike Squads serve as either a scout force used to quickly gauge the enemy force or as vanguard to swiftly attack enemy formations and allow the infantry squads to mop up the survivors. They are often paired with Land Speeders for these tasks. A distinct variation exists called the Attack Bike, which incorporates a sidecar to cart around a heavy weapons guy armed with either a heavy bolter or a multi-melta. These can be fielded as squadrons of three, or embedded into the usual bike squads. A Bike is also an option for all your generic HQs, offering a decent compromise between speed, survivability and price. The book also warns you that you can't ride it while wearing terminator armour.
Scouts also like to ride bikes to help with their scouting. While their older Marine brothers are geared for assault, Scout bikes are more geared for the fighting retreat and harassing the enemy with clustermines and other goodies. They're also used as a way to swiftly deliver a locator beacon where you need it.
Bike Squads often vary depending on the Space Marine Chapter they belong to. The White Scars use Bike Squads, echoing Jaghatai Khan's use of mounted cavalry during his conquest of Chogoris. The Dark Angels have devoted their entire Second Company into utilizing Bike Squads and Land Speeders as the Ravenwing, echoing the mounted knights of the Order on ancient Caliban who hunt
the Fallen Angels TRAITORS FROM THE NINE LEGIONS AND SUBSEQUENT RENEGADE CHAPTERS, NONE OF WHICH INCLUDE THE SONS OF LION EL'JONSON. The Space Wolves Bike Squads are known as Swiftclaws, who are operated by Blood Claws (the Chapter's equivalent of Assault Marines) and serve as a vanguard force. Their numbers are few, as the Space Wolves consider Bike Squads an overly headstrong tactic (though riding in on a Fenrisian wolf is okay).
The Outrider Squad is the Primaris Space Marines' equivalent of a Bike Squad. It closely resembles the bikes used by normal Space Marines, but with up-added armor and a lower profile. Outriders are most commonly used to advance ahead of the main lines and to guard the flanks of larger formations.
In battle, they perform lightning-fast hit-and-run attacks on enemy strongholds and are employed to chase down any who would seek to escape. When additional firepower is needed, an Invader ATV would assist these guys in combat. Much like their smaller cousins, they have absolutely zero ground clearance so only the Emperor knows how they go over a speed bump, let alone rough terrain.
Their Raider Pattern bikes are equipped with frontal Bolters while they themselves wield Chainswords and Bolt Pistols. The performance of the bike is effective as long as the rider can dominate its bellicose Machine Spirit. On an additional and hilarious side note, the Primaris Go-Kart also looks like two Outrider Bikes being fused into one.
With their various special rules giving them six(!) attacks on a charge with a base ap -1 these things have immense RIP AND TEAR potential which combos well with their niche of "only fast primaris assault unit". Who knows what their eventual options will be (if any) when their kit comes out but their current form is clearly intended to be used with the White Scars chapter tactics. Their real weakness is their average strength of 4, though even this can be mitigated by a fancy new bike chaplain throat-singing the White Scars litany of wound re-rolling.
The Imperial Guard also has their own bike squads, but they are naturally not as awesome as the Space Marine bike squads. Rather than having two storm bolters, our Guardsmen have...nothing. While this may put the poor men in t-shirts at a disadvantage, the Imperial Guard's bike is more useful for scouting purposes than as a fast attack vehicle. They are often made for Rough Riders who don't like maintaining the horsies and prefer a more simple and cost effective mount.
They also look more like regular old motorcycles.
On tabletop, since these guys are grouped as 'Rough Riders in all but name', they will likewise have the same rules even though what their
riding driving on is not an animal in the slightest and they usually carry a Laspistol rather than a Hunting Lance. But because GW is lazy (and after Chapterhouse, they're not going to make rules for models they never made), they decided to why not make these guys Count as space cavalry.
In the newest update, 8th Edition's been kinder to Rough Riders than 7th Edition was. Their hunting lances are no longer one use only, being S+2 AP:-2 and do D3 damage, but still only when they successfully charge. Cheaper, twice the wounds, and now their flak armor might actually do a damn thing, though not against most multiwounding weapons. In addition, they may perform Flanking.
Similar to that of the normal Imperial Guard variant except these guys serve a four armed dude rather than a two armed one. Like the Imperial Guard, the Genestealer Cults use their bikes primarily for scouting purposes rather than as a fast attack vehicle.
The bikes they drive upon is called the Dirtcycle which like all Genestealer equipment and machinery, is stolen from the Imperium of Man. The Dirtcycle itself is an Imperial two-wheeled Atalan-class exploratory machine. These rugged machines have a robust frame and shock-absorbing suspension array. Its engine can run on multiple fuel tools and can mount heavy loads. Like the Lasgun these vehicles are cheap and easy to manufacture.
The drivers of the Dirtcycles often carry simple one-handed weapons like pistols, hatchets and axes to do some fast hit-and-run attacks against unsuspecting foes. Granted, it is only effective against lightly armored opponents, but they still get the job done.
Unlike the Guard's, the Genestealer's also have a secondary purpose for these vehicles. During times of uprisings, the Genestealer Cults combine the speed and nimbleness of the Dirtcycle with that of the more stable and heavily armed Wolfquad to form a combined arms approach; the Dirtcycle is often used as an outrider by Genestealer Cults' Atalan Jackals formation.
Simply put, for the first time the Genestealer Cults finally have access to fast attack units.
Chaos Space Marines
The Chaos Space Marines as usual have their eviler and more edgy counterparts to the loyalist Space Marines.
In a similar way to regular Bike Squads, Chaos Space Marines also utilize their Bikes for reconnaissance and scouting tactics, although with the usual, more sinister twist. They are also
often always covered in spikes and spurs as per Chaos Space Marine tradition in order to slice enemies up as they zip by. Unlike loyalists that gravitate towards the "horse archer" role, heretics are composed more like shock cavalry, having free chainswords by default and access to melee-boosting marks and icons. The huge number of spikes and blades they attach to the chassis actually gave them an extra attack in earlier editions. Nurgle-aligned Chaos bikers back then were a horrifying staple with their Toughness 6 basic statline, while Tzeentchian bikers had the usual invulnerable save.
Chaos Marines do lack the Attack Bike option because the thing was rare during the Horus Heresy. Presumably the ones they did have were eaten by Chaos Spawaaaaaaaaaauuuuuuuuuuuuuuuuuuuuuuugggggggggggggblblblble.
Perhaps due to the noise, speed, and profile, Slaaneshi Marines have always had a special fondness for the bikes, in every edition they've had them. In 3rd Edition, bikers following Slaanesh could change their bike-weapon for Sonic Blasters for added Rock'n Roll, and could replace meltaguns in their Biker squads with Doom Sirens for maximum noise. The Violators are a Chaos Warband well-known for its use of Chaos Bikes and sonic weapons. And lastly, of course, there is Doomrider, the quintessential example of a Slaaneshi CSM who uses his bike's immense horsepower and cocaine to simultaneously pleasure himself. The rough vibrations on the bike's driver seat pleases Doomrider greatly.