Raptors are Chaos Space Marine Assault Marines. Or, at least, they were at first. Then Games Workshop decided that they needed something original about them and made them separate from the Legions, turning them into mercenary warbands, or "cults" who don't worship the big four, instead worshiping the "Raptor God". And now they're Chaos Assault Marines again, with at least one in the Black Legion who worships Slaanesh. Games Workshop sure is indecisive.
And despite what people might tell you, the Night Lords are NOT the "Raptor Legion" (Except now, where Night Lord legion traits stack with Raptor morale debuff, bitches!). Though they are creators of Raptor squads and distinctive Raptor armour with clawed feets and large engine intakes, so Night Lord bands often have a large amount of Raptors in their ranks. The relationship is more that between the Word Bearers and Possessed or the Iron Warriors and Obliterators (indeed Siege of Castellax features IW Raptors) - they are distinctive motifs of the legion, yet not really so abundant as to define the group entirely, and in this case not having much real political power, instead being treated by "normal" marines as little more than beasts. Quite rightly when you consider how they deteriorate into mad killers, even losing the power of speech over time. The Night Lords still eschew the whole Chaos worship thing and only care about material/temporal power, so they probably find the Chaos-crazed Raptors more perverse than anyone else does.
Speaking of Raptor armour, it seems GW retconned the old awesome one (with predatory helmet, brutal-looking jump-pack and bird-like fanged legs) for the lame heresy-era armour with two beer kegs on the back and stupid little blades on the feet. Not like it is the first time GW replaces cool model design with derpy one. Still, the newer kit gives you a few valuable things for modellers - chaos chainswords that don't look like total ass, sweet-ass horned Maximus helmets and the kitbashers's holy grail in the form of a left-handed plasma pistol.
They were present in the THQ Fire Warrior game, where they actually did screech like birds (and boy did it get on your nerves after a while let me tell you....). Despite them ostensibly being terrifying for a Fire Warrior, being essentially fast space marines who would engage you in melee (all of which give Fire Warriors difficulty), the Raptors are almost sad in how easy they are to beat because once you start facing them, you have the Rail Rifle and can oneshot them in any given engagement. Just don't miss. (Which, in the PC version, is far easier said than done, considering the wonky hit detection).
The make another appearance in Dawn of War 1, as Chaos' basic jump-assault infantry, where they essentially function as a slightly gimped version of the loyalist assault marines, as while they can take frag grenades to disrupt infantry (at least those that can be knocked back),
the lack of a squad leader upgrade means lower morale, lower squad size, and no power-weapon attacks. They have a raptor champion upgrade and can also be equipped with flamers.
Do not confuse with the Raptors, a loyalist Space Marine chapter.