Land Speeder
Not to be confused with the "landspeeder" from Star Wars.
The Land Speeder is a light reconnaissance and strike vehicle used the the loyalist Adeptus Astartes. None of the traitor legions use them because grav technology has issues in the constantly-changing physics of the warp. Or something. Fuck if I know. Land speeders came from an STC found in M31 by the esteemed and venerated OG Techno-Archaeologist Arkhan Land (of Land Raider fame). It just so happened to be that the space marine legions were looking for a paper plane to ride around in fucking with their enemies, so the land speeder took off! During the Great Crusade the Imperial Guard (at that time the Imperial Army) made use of the Land Speeder, but by the 41st millennium only the space marines get them because grimdark technology is being lost we forgot how to anti-grav everything sucks yadda yadda.
On the campaign trail land speeders are primarily used as light reconnaissance vehicle, used to scout out assorted information when marines don't have air superiority to scout things out the sensible way. Occasionally they're used in battle as fast attack strike vehicles, especially by the Raven Guard and their descendant chapters. Land speeders rely on speed and firepower to survive in battle, ideally jetting in and destroying whatever they're attacking before it can retaliate, though more often than not they end up exploding in a glorious ball of flame after blowing up a few things. The default Land Speeder is armed with a heavy bolter, but they're often up-armed with rocket launchers or assault cannons.
Variants
Land Speeder Tornado The Tornado is an up-armed dedicated fire-support Land Speeder variant. The standard loadout is a heavy bolter or heavy-flamer and an underslung assault cannon. All the best parts of a typical land speeder with some extra anti-infantry firepower. Fluff-wise this is the most common landspeeder employed by the Adeptus Astartes.
Land Speeder Typhoon The typhoon is a land speeder variant designed to serve as a heavy-support platform. It's a landspeeder with a big goddamn missile launcher taped to the back. It often times has a multi-melta. It blows shit up. Moving on!
Land Speeder VENEGANCE!!! The land speeder VENGEANCE!!! is a dakka version of the typical landspeeder with a somewhat ridiculous design. Instead of the underslung assault cannon most landspeeders have, it has an armored gun cage on the front that a marine sits in. For some reason.