The mode plays similarly to multiplayer Rush, the objective being (besides simply surviving) for your squad to capture one of several flags while progressing along a fairly linear map layout. In fact, as noted in the mode's announcement, those maps will be ones that already exist in the game, but they have undergone some tweaking, primarily seasonal and time-of-day changes. The maps are: Atacama Desert, Nelson Bay, Isla Inocentes and Valparaiso.
In action, the mode feels very much like the single-player component would, had DICE built four-player co-op into it. Unlike multiplayer matches, it relies heavily on scripted events and enemy spawning, with some arriving on-board patrol boats, depending on the map. I found that not only is using squad tactics and maintaining situational awareness key to staying alive, but getting through the stages really requires a well-balanced squad. I'm not saying it's impossible to play as four assault class characters, but things definitely went smoother when I played recon, clearing a path for my squad mates using artillery strikes and long-range sniping. Overall, it was a lot of fun and a good challenge.
Another note about the mode: unlocks from multiplayer carry over, so your character is going to be as well-equipped and capable here as he is in the rest of the game (and vice-versa, since you still accrue XP in Onslaught). This also means that as you advance in multiplayer, you should theoretically be able to place higher and higher on the leaderboards when playing with similarly ranked partners.
The release date for the Onslaught DLC is still officially "to be determined," but an EA rep I spoke with said he expects it to hit in early June. Pricing is yet to be decided, but I wouldn't be at all surprised if EA was waiting to see how well Modern Warfare 2's "Stimulus Package" sold at its price point before setting something in stone.