The Ravenwing is the name used by Dark Angels and their successor chapters for their Second Company. While the Second Companies of Codex Astartes-compliant chapters are "Battle Companies" nominally consisting of six Tactical Squads, two Assault Squads and two Devastator Squads, instead the Ravenwing specializes in using bikes, Land Speeders and aircraft.
Company Composition: i.e: ALL the Bikes
Way back in the darkness of 3e, the Dark Angels' traditional battle companies could include "normal" bikes of their own just like every other chapter (as an alternative way of deploying assault marines) though it seemed that as time went on, every single biker and land speeder pilot in the chapter and all of its successors got promoted into the Ravenwing (or equivalent) and by 6e the Dark Angels lost count of how many members the second company actually has in it (assumed to be more than 100) but that's what you get when you steal a chapters worth of bikes and cram them into one company. From the newer fluff; the 7e codex states that Dark Angeks "claim" that the Ravenwing maintains codex company strength and lists ten squads of ten men, but fails to include the number of Black Knights and even goes as far as suggesting that the Dark Angels deliberately obfuscate the true number so the Adeptus Terra doesn't have a clue.
In terms of skill; Ravenwing compete with White Scars for the best bikers in the Imperium. Whilst the White Scars probably win out as an entire chapter dedicated to mounted combat rather than just one company and have better tabletop rules representing the average bike-riding individual. However, the Ravenwing's elite Black Knights are just superior in every way and have plasma-wielding bikes, rending pick-axes and rad or stasis grenade launchers (Grey Knights lost access to funky rad grenades, leaving the Dark Angels with sole access to these cool items) and their leader: Sammael has an endless supply of the "last" jetbike in the Imperium, kitted out with a plasma cannon.
Ravenwing also have access to pimped out Land Speeder variants not seen anywhere else: The Land Speeder Vengeance which has a Large blast plasma array and an overhyped reputation for glancing itself to death, and the ever useful Darkshroud which has bits of Caliban modded onto it which blanket friendlies with darkness (hence the name).
Ravenwing Attack (Bikes and a single Land Speeder as one formation) and Support Squads (up to five regular speeders rather than the traditional three being accompanied by a Darkshroud or Vengeance) may be seconded to a detachment of Dark Angels, or they may deploy as an army in their own right with their own detachment.
In traditional codex chapters, the pilots of space marine aircraft are techmarines (sometimes; it's complicated). Amongst the Unforgiven chapters however, all of the pilots of the Dark Angels aircraft are members of the second company instead. Unfortunately the supposed benefits of doing this are dubiously represented on the tabletop, since both of the Dark Angels aircraft are inferior to codex aircraft and the distinction between Ravenwing fighter aces vs Techmarine pilots are only detailed in the Crusade of Fire supplement which many people may not own. Giving them inferior abilities and kamikaze maneuvers. Seriously! one of the Ravenwing ace pilot traits gives them a teleport homer, the Deathwing will most likely already be on the table before or at the same time as the pilot actually arrives due to Deathwing assault.
In the 7th edition update, all Ravenwing Aircraft and Land Speeder pilots (as well as all Bikers) are given the ability to reroll cover saves when jinking, so there is finally a bonus. The was already represented in the Deathwatch (RPG), where dedicated ravenwing pilots are at a slight advantage over nearly all other chapters pilots (including White Scars) with the ability to use evasive action more freely than others.
Role & InductionDeathwing, is tasked with hunting the Fallen Angels, though the Ravenwing know far less about their targets; as far as they know, the Fallen are simply heretics who happen to bear a grudge against the Dark Angels, and enjoy spreading lies about the Chapter's history. When the Inner Circle expects to encounter lots of Fallen, they may deploy the Ravenwing and Deathwing together: the Ravenwing chase down the heretics, and then use their locator beacons to call down the Terminators.
They got fluffed up considerably in the aptly named "Ravenwing" novel by Gav Thorpe. The members are inducted from the most curious space marines: in fact the protagonist of the story is simply transferred over for asking the seemingly innocent question of "how did the Lion die?". Therefore clearly his ultimate fate is not a matter for casual discussion within the chapter so most of the common variety "green" Dark Angels obviously just accept what they are told off hand and don't bother thinking about it too deeply.
In fact the Ravenwing is part of the mechanism that keeps the secret of the Fallen from within the chapter itself, racing ahead of the rest of the army and making certain that nobody gets exposed to uncomfortable truths that the Chaplains don't want to get out. Which is partially an unfortunate thing, as members of the Dark Angels will never be promoted to the higher ranks unless they can be trusted with the truth, which has far less to do with skill than it does with the personality of the marine in question. So some Dark Angels may be stuck in green armour no matter how competent they are as soldiers/warriors and might not quite realise why they aren't being "promoted" and get a bit resentful about how much freedom & glory that the Ravenwing/Deathwing are getting. But that's what the Inner Circle would prefer; the fewer who know the truth the better.
However, rather than being given an outright lie for an answer when they start asking questions, the members of the Ravenwing are slowly inducted through several circles within the company itself, gradually coming to the truth of the fallen as they prove themselves more and more worthy in the eyes of their superiors, either that or the Ravenwing is a dumping ground for initiates who need closer observation (and potential removal). There have been documented cases of promotion to the Deathwing without having any prior association with the Ravenwing at all. Obviously having proven their trustworthiness without any shadow of doubt by other means...
Eventually, when members of the Ravenwing become promoted to the rank of Black Knight they become privy to the truth of the Fallen and what they represent, if perhaps not the entire truth of the betrayal of Luther. Whilst those from the chapter who have direct contact with the Fallen will be segregated into the Deathwing instead, acknowledging that it is not strictly a promotion to be proud of; rather it is now an additional burden they must bear.
VS. White Scars
Theres currently a great deal of debate on whether or not White Scars are better than the Ravenwing. The crunch mirrors their fluff match up pretty well.
Ravenwing are a smaller, more expensive and overall more niche force, whereas the White Scars are a far larger, individually cheaper force of uprated bikers. In terms of per-model costs and abilities White Scars are undoubtedly better, being able to spam far more bike models that are not only better rules wise but also cheaper. Thats only counting the basic bikers though, as the "Ravenwing" also encompasses the Chapter's land speeders and flyers, which benefit from the similar bonuses. Furthermore, Ravenwing's Black Knights are far better than any bike unit the Scars could field with the Vanilla codex and Sammael beats Khan in every way. DA can also take land speeder squadrons of 5 compared to the vanilla's 3. Further, Ravenwing also support the rest of their army better, such as being able to Scout forward and Outflank without the need for special characters, and carrying Teleport Homers to guide Terminator squads in. It really comes down to your preference.
...and then both forces were updated in 2015. with Ravenwing getting rerollable Jink and overwatch at BS-2, base. This just gets cheesier as you add in the formation bonuses where Ravenwing boost each others ballistic skill or can overwatch for each other, then if taken as part of a Lion's Blade Strike Force they get upped to BS-normal with Land Speeders gaining damn Supporting Fire. If taken as part of a Ravenwing Detachment instead they can turbo boost on turn one to get a "free" Jink on the enemy's next shooting phase, i.e. full Ballistic Skill on turn two!. Deathwing Strike Forces also benefit from being deployed with Ravenwing by being able to choose to pass or fail reserve rolls when accompanied by certain formations (which also have the aforementioned Teleport Homers).
What did the White Scars get? With the release of the new Kauyon supplement, which was later rolled into the Angels of Death supplement with the other non-ultramarine chapters, the White Scars have just received a Decurion-style detachment all to themselves. Now White Scars in this formation are arguably the fastest and most mobile space marine chapter in the game, and can again field an all biker army again with all the lovely bonuses that come with these new formation-detachments. The formations in this detachment are also brutal as well, with tons of fluffy formations and abilities that you cannot find in the regular Gladius Strike Force (while still allowing use of the formations available in the Gladius). However it lacks the free-transports ability (which is unfortunate) it more than makes up for it with abilities that benefit a fast moving hit-and-run army.
In summary though, Ravenwing are really about bringing synergy with a combined force, rather than being the force itself. Where the Scars are really about the bikes and aren't focused on much else. If you want a larger force of exclusively bikes then Scars are better. If you want a smaller support force with great special units then Ravenwing are better.
GW knows how to make those bad boys look awesome, which they most did by stealing the wings off a polish Hussar, but still.
See that? You seein' that? Mere words cannot describe it's awesomeness! That's what happens when the Ravenwing work with the Deathwing
The last jetbike in the imperium... if it wasn't a different pattern from Heresy-era Imperial jetbikes and it didn't appear thousands of years after the Horus Heresy... and that it magically pops up whenever it's been destroyed... Or you know, it may be a Bullock pattern's variant. it looks awfully similar, just saying.