Basically, it's a massive tactical FPS a la MAG which features huge infantry and vehicle combat, in-game currency and economy, and a ridiculously complex MMO-style progression system. And when I say ridiculously complex, I don't mean the kind of complexity that Call of Duty seems to love boasting of its customization system, I mean stare-at-the-skills-tab-for-15-minutes-and-still-not-be-able-to-figure-out-what-the-fuck-is-going-on complex. You learn the system though, and in doing so discover that the game is very geared towards specialization. You can only really be effective at a particular skill set if you focus on it, so there are some people who've put all their points into combat, others who've done the same with hacking and mobility, and others who have spent all their skill points on piloting dropships or tanks.
The game's more than solid as a large-scale FPS, but where it really shines is in its interaction with EVE Online. Both games run off the same server, dubbed Tranquility, and both deal with the same spaces in real time. Massive corporations controlled by Eve players (And potentially Dust mercenaries, as both can create corporations) will battle over districts, planets and ultimately whole galaxies for resources, power or strategic advantage, and the exploits of Dust mercenaries will be at the forefront of it all. CCP has five and even ten year plans for introducing more and more content to the constantly fluctuating game, and some of the stuff they've been proposing is goddamned unheard of. Once Eve and Dust are fully integrated, starship battles by Eve players will be occurring above the Dust battles, and all in real time. Dust players will be able to call support from Eve players on PC in the form of ordinance, which will range in power from tactical orbital strikes to planet-scorching blasts. At some point, the developers have suggested that they may be able to implement a drill-like pod that can be fired from one massive cruise ship into the other, effectively allowing a deployment of Dust mercenaries to invade and attack enemy ships.
TL;DR: Dust 514 may be the most ambitious shooter ever, and it's free to play.