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 (and simply they don't have anymore the tech to maintain them as well that they simply lost a lot of them), though some of the newer warbands that were once loyalist Chapters do have them and use them. 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. This only sounds dumber when faced with the fact that numerous bits of fluff and literature (Ravenor, Eisenhorn, and Ciaphas Cain in particular) make mention of air-cars, air-bikes, speeders and other such vehicles in civilian use which are presumably anti-grav (of course this could just be because the technology that is still known to the Imperium simply isn't up to the rigors of combat, similar to how cars existed for several decades before being reliable enough to work as military vehicles).
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 Land Speeder 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 Land Speeder with a big goddamn missile launcher taped to the back. It often times has a multi-melta, and is effective as a skirmisher that can rapidly outflank and destroy enemy vehicles.
Land Speeder Tempest
The Tempest is a specialized Land Speeder variant that differs from other Land Speeder in two main ways. Firstly, instead of having an open cockpit, the entire crew cabin is enclosed and armored, thus helpfully preventing the pilot from being shot in the face by some punk on the ground with a heavy stubber. Secondly, the Tempest is loaded up with better overall armor, internal missile launchers and a beefier engine, and serves as more of a fast attack support craft then other more fragile Land Speeders.
Land Speeder Storm
This variant is modified to act as a transport for small Scout teams to infiltrate an area and can provide fire support for them with its heavy bolter and Cerberus missile launcher. Additionally, it can force teleport homers to malfunction, allowing it to interfere with deep strike attempts as well.
Land Speeder Vengeance The Land Speeder Vengeance is a much heavier version of the Land Speeder exclusive to the Dark Angels and their successors. Armed with either a Plasma Storm Battery that can tear apart TEQs or an ancient relic that repels enemy fire, it is a potent asset for the Unforgiven that they inevitably have refused to share with anyone else.
On the Tabletop
Crunch-wise the Land Speeder is a fast skimmer made of wet cardboard. With AV10 all around and 2 Hull Points you better keep it far and away from the enemy's weapons, or deep strike it to make a suicide run. For the first method you should kit it with a Typhoon ML and either a Heavy Bolter or a Multi-melta, while for the latter you can choose a combination between Multimelta and Heavy Flamer.
In a Gladius it can fill the Fast Attack role (if for some reason you don't want to take the almighty Bike Squad) quite cheaply. Give it a Heavy Flamer, keep it in cover the entire game then on the last turn zip it halfway across the board to capture or contest an objective (remember it has ObSec from the Gladius).
A couple of patterns stand out: the Land Speeder Storm can carry a half-squad of Space Marine Scouts, while the Land Speeder Tempest is fully-enclosed and more armored. The Dark Angels also employ their own versions of the Land Speeder, which have larger thrusters, a large turret on top, and a small gun cage on the front end. Which makes you wonder what they were thinking, attaching a tiny turret manned by a massive, heavy marine to the front of an extremely fast and fragile skimmer...