The original game has all the regular content.
Entrenchment has everything the original game had, plus starbases (I could never go back to playing without starbases; so much awesome), minefields, special static-defense upgrades, and assault cruisers.
Diplomacy has everything that Entrenchment had plus a revamped diplomacy system, envoy cruisers, diplomatic pacts, missions, enhanced pirates, and a bunch of other goodies.