Hello and welcome to the forum! 
Please allow me to give you a quick history lesson about Sins of a Solar Empire:
- Sins of a Solar Empire - Original Game
- Sins of a Solar Empire Entrenchment - First Mini Expansion to Sins of a Solar Empire
- Sins of a Solar Empire Diplomacy - Second Expansion to Sins of a Solar Empire, adds even more content over Entrenchment.
- Sins of a Solar Empire Rebellion - First Stand Alone Expansion containing everything from above + a ton of new content
- Sins of a Solar Empire Rebellion FORBIDDEN WORLDS- Small but good DLC for Sins Rebellion only.
Sins of a Solar Empire TRINITY is not a game itself.... it is merely a collection of
- Sins of a Solar Empire
- Sins of a Solar Empire Entrenchment
- Sins of a Solar Empire Diplomacy
Sins of a Solar Empire Rebellion is the first stand alone expansion, it contains EVERYTHING from all earlier games and is the only thing you normally want to buy!
I would also recommend getting the DLC "Forbidden Worlds" for it.... last time I checked it costed only 5 $ which it is well worth imho.
The only drawback is, that unlike TRINITY or Diplomacy.... Sins of a Solar Empire Rebellion requires Steam.
Sins of a Solar Empire Rebellion short facts: (in no particular order)
- Excellent developer support
- Support for up to 10 players (Note: As the game sadly does only support a single CPU core, such games tend to run rather slow, especially with AI)
- Full LAN support
- Excellent modability.... and many awesome mods to choose from already
- Runs on XP, Vista, 7 and probably 8.
- Epic space panorama
- much more
- 3 different races
TEC: The Trader Emergency Coalition was originally created to deal with the Vasari threat. By utilizing their economic power, the TEC have managed to survive by being industrious and adaptive.
Advent: The Advent are a highly advanced civilization that rely heavily on spiritual, psychic and cybernetic technologies. Long ago they were outcast by the Trade Order and so seek to destroy the TEC.
Vasari: A once mighty empire of countless worlds, the Vasari have been travelling for 10,000 years to escape some unknown terror. Now in Trader space, they've been embroiled in a long war.
Play as TEC and use their enormous economy and industrial might to crush your enemies with an armada of highly durable ships. Play as Advent to exact revenge on the TEC for your exile and make your enemies succumb before your mighty synergistic psychic powers. Or play the Vasari.... cunning aliens that use superior technology and dirty tricks that your enemy will never suspect in time.
- With Rebellion, the tree races split into 6 subfactions, which are defined by special researches and their respective Titans:
ACTUAL INGAME GRAPHICS!
Colonizing planets is a rather simple thing in Sins of a Solar Empire.... you need a colony ship or a Capital Ship with Colonize ability to found new colonies. Some planet types require research to be done before they can be colonized.
Planet types are different, for example Terrans have high population, Desserts have a ton of logistic slots (needed for space stations needed for research and trade) and Ice have a lot of crystal extractors.
There are several more planets in the game.... the DLC for Rebellion "Forbidden Worlds" adds a few more as well as many new planet bonuses.
Planets need to be developed in various ways.... although that is done with the click of a button.
You dont go down to the planet to build single structures..... you just develop the entire planet from orbit.
Sins of a Solar Empire is a RTS focused 4X game.... the research tree is comparable limited to Gal Civ.... and pounding your enemy into oblivion is usually the best, but not the only way to win. You can win with Diplomacy and Research too.
Diplomacy system is good.... but naturally on a far lower scale than Gal Civ ones.
The best way to befriend other factions? Bomb their enemies back to the stone age. 
Planetary Invasions? Not in this game.... and why bother with sending troops when you can just nuke the living shit out of it from orbit anyway?^^
If it does help..... I first played Sins and only later learned about Gal Civ..... I enjoy both games now.
Others will have more info, but imho Sins of a Solar Empire is an awesome space game.
