BOS Wars is a futuristic real-time strategy game with a strong resemblance to the Command and Conquer series.
You start with a very small force, and must build your army up quickly to deal with marauding enemies. But you've only limited resources, and so you must also expand your base to harvest more energy, increasing your capabilities.
If you can get the balance right - build up your base, but also keep enough units around to fight off regular attacks - then you should be able to build a large enough force to march around the map and defeat your opponents. But beware, it's not easy. At all. Complex gameplay and smart AI means Bos Wars will take a very long time to master.
Once you've learned the basics, though, there's plenty to keep you entertained here. Like a campaign, plenty of game maps, and more available for download. You can create your own with a built-in editor. And multiplayer support allows you to challenge friends to a game, either across the internet or on your local network.