Official Description, does a better job than me.
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Sins forums
Sins of a Solar Empire is one of the most epic games I have played, it is currently broken into three separate games,(Grouped together its called Trinity) however this thread will only be dealing with SINS: Rebellion as it contains all the features of the previous games, so someone who is interested in Sins of a Solar Empire won't be spending their money buying Trinity when all its features are included in the newest game being made. (Rebellion)
This game does have a few issues (for newbies), its steep learning curve may put off some people, but I highly recommend this game to anyone who enjoys a deeper gaming experience.
One final drawback is the lack of a dedicated story line, however the manual and assets in the game do help layout a basic story.
Dedicated lore here.
Rebellion is currently in an open BETA stage, so those who preorder can get access to it. Rebellion also requires STEAM to play, so those who dislike such things won't be interested.
But having played it I can say even in BETA stage its definitely worth the money and it will be pure win once fully finished, which will be June 12 2012.