Despoiler Squad
An old type of Space Marine infantry formation, Despoiler Squads were used extensively during the Great Crusade and Horus Heresy and were a sister unit of the Breacher Siege Squads.
Overview
When conducting boarding actions or trench clearing operations, Space Marine Tactical Squads would sometimes forgo their standard Bolter in favour of a Bolt Pistol with attached Chainblade, Chainsword or a Heavy Chainsword, allowing for greater manoeuvrability in the tight confines of a trench or the cramped corridors of a ship.
Therefore, think of Despoiler Squads as an Assault Marine without the jump pack, or a Breacher Squad that sacrifices defence for balls-out agility and speed.
They tend to wear Mark III armour, since it was designed specifically for the job they were doing, although some would wear the more modern Maximus-pattern armour. And yes, they have nothing to do with a certain armless failure.
Crunch
Horus Heresy 1.0
In the tabletop the Despoiler Squad are basically the same as the fluff, Assault Marines without jetpacks. Offers a fine balance between Tacticals and Assault marines. The Sons of Horus seem to really like the Despoilers, having created their own variant called the Reaver, which are comparable to Legion Veterans with 2 Attacks base, as well as Outflank and Precision Shots and Strikes. Precision Shots and Strikes lets Reavers pick off sergeants, special and heavy weapons users and Apothecaries, which is very useful. Unlike Veterans, they don't come standard with a bolter. Their wargear is assault-oriented: Ranged weapon choices are Bolters/Combi-Weapons with Banestrike shells for AP3 Rending or Volkite chargers for extra wounds (so your longest ranged attack is 18") and one in five can take a special weapon. Any model can have a Chainaxe for just 1pt, or a more expensive Power Weapon or Power Fist. They can also come with a jump pack for an extra 50pts, making them essentially, Assault Marines in all but name.
Horus Heresy 2.0
Tactical Marines, but for melee. They lack Bolters but all come with Bolt Pistols and Chainswords, the latter of which can be swapped out for a Heavy Chainsword for fighting Automata and monsters, or for a second Bolt Pistol to make a squad of pseudo-Destroyers. Every model in 5 can take a Power or Charnabal weapon, and the same Marine or another 1 in 5 can swap its Bolt Pistol for a Hand Flamer, Volkite Serpenta or a Plasma Pistol. The unit has Line and Heart of the Legion, as well as Spite of the Legion, which gives them an extra attack in the turn they Charge a unit that is Pinned, Falling Back or doesn't have any Chosen Warriors or Characters (including squad Sergeants), which makes it situational. Luckily, you don't have to declare it like Fury of the Legion, so it's a nice bonus under the right circumstances.
Despoiler Squads can work very well paired with a Tactical Support Squad. Take Rotor Cannons on the latter and have them act as suppressing fire on whatever the Despoiler Squad is going to charge that turn. With luck, you will pin them and thus trigger Spite of the Legion. Alternatively, take Recon/Scout snipers; you can either pin the enemy, or kill their Sergeant and claim Spite of the Legion regardless.
Spite of the Legion does not specify the Unit sub-type your Despoilers are charging. RAW, you can usually claim Spite of the Legion against Dreadnoughts and vehicles because they're not Characters. This won't help much with your normal chainswords, but 1 more attack each from your 4 Power weapon guys and your Power Fist sergeant could be the difference between killing that Leviathan or not.