If you've played any of the Command & Conquer games, or Dune II, then either case should leave you to feel right at home. Dune veterans will understand the structure tree and units better, but will be confused by the controls. C&C players, on the other hand, will understand the controls better, but will take some time acclimating to the 3D terrain and the differences in resource management.
In the world of Dune, there is little of value save the precious spice known as Melange. Melange is a drug that is used in a variety of ways, extending lifespan, furthering the intellectual powers of the religious order of the Bene Gesserit, and providing the user with the means to bend reality to his will, allowing for space travel. It is fundamentally the most powerful substance in the universe, and thus commands a very high price among those who desire its power. Melange, known as spice, is filtered from the sands of Arrakis by harvester machines run by each of the houses, and exchanged for credits used to purchase more equipment.
Building an army does take money, of course, but fundamentally starts with the buildings. At the bottom of the support chain of every base is the Construction Yard, from which all sub-structures are built. If you can destroy an enemy's construction yard, you effectively prevent him from rebuilding his base, and thus turn the tables in your favor. Of course, the construction yard cannot stand on its own. Vehicle factories and barracks are constructed to train new units to defend the front, as well as push it forward. You must also have the appropriate facilities in place to separate the spice from the sand, so a refinery or two is in order. And finally, no base can run without electricity, so aspiring commanders must install Windtraps, huge electric generators powered by turbines that feed off the second-most abundant thing on Arrakis - wind.
On the surface, it seems that the game is just a clear-cut case of resource management and position defense, but there's also power management to be involved as well. Different structures require different amounts of power, and if you overload your power grid, then some structures will operate less efficiently, and others won't operate at all! Furthermore, if your windtraps get damaged, their output drops, necessitating a close eye on base maintenance. I can't count how many times I went to build a turret only to shut down all the rest of them because I was sapping too much juice.
Also, Dune plays resource management slightly different from other RTS games. In C&C, for instance, funds were generated by refining the toxic ores which periodically regenerated. In Starcraft/Warcraft, the resources on each map are fixed, but your peasants/drones/SCVs/probes were generally close to their homes, meaning that you usually only sought what was close to your town. In the world of Dune, however, Carry-alls are used to ferry the harvesters to and fro, meaning that you could effectively harvest your opponent's spice fields if you so desired. Of course, harvesting so far away from home takes longer, and poses more of a risk to your Carry-all and harvester, but things like this happen on Arrakis. Most enemies won't think twice about stooping to shoot on an enemy harvester, but typically they're so far away that it's not really worth it, and so spice collection is more of a game of "grab it as fast as you can".
Where do you want to rule today?
While other games say they have a branching storyline, Dune produces one very effectively while meshing it with the tactical battle management system. Each of the three houses picks a region to take during its turn, which means that while you're attempting to invade the lands of your rivals, they might be fighting it out between themselves, or they might both pay you a visit, requiring you to defend your land. Or, and this is one of the most overlooked ideas I've seen in an RTS, you can simply forfeit the region and let it get taken over by your rival. If you're just looking to blast through the game, or you want to make things longer and more difficult for yourself, then you can decline the opportunity to defend your lands and move on. Furthermore, whenever you are on the attack, you have the opportunity of picking the point of entry based on what bordering regions you control. This removes the concept of a scripted mission and makes things a little bit more random. Add this to the sheer number of missions you face, and you're in for a long but satisfying ride.