Star Realms: Colony Wars is officially an expansion for Star Realms, a fairly popular and quite good deck building game. While there are a number of small expansions for Star Realms, Colony Wars is notable because it is actually another whole game.
Star Realms initially had a small marketing problem; it was a two-player base deck for a game that worked really well for 2-4 players. You could buy the base deck and then add on a two-player starter deck expansion to accommodate four players, but then you would run out of cards too early; you could buy mini expansions for more cards, but these were comparatively expensive and didn't give you many cards. Or you could do what most fans did, and buy two base games. Compared to games like Ascension or DC Comics Deck-Building Game this was a confusing mess, and it ended up making what was supposed to be a comparatively cheap deck-builder into a confusing and not-so-cheap mess.
Enter Colony Wars. This is exactly like the base Star Realm box, except that all the cards are different. Well, almost all. The starter-deck cards for two players and the score-keeping cards are exactly the same (Explorers are also the same); but the ships and bases for 'Trade Row' are all new. They are not particularly different or special in any way. No new game mechanics are added. This just takes the base game from 2 players to 3-4 players without requiring you to double-down on the base set.
Because of this, if you have not yet bought Star Realms and would like to, you may buy either the original or Colony Wars; it makes no difference to game play.
This is a good and necessary expansion, and I am very glad that they made it. However, it is not clear from many game reviews exactly what it involves. Some of the mini expansions, such as Star Realms: Gambit and Star Realms: Crisis: Events add new and spiffy game mechanics. While these smaller expansions include only about 20 cards each, if you are happy with the two player game they are much better value.