Freight: Disband a freight unit in a city and you'll get 2/3 of its production cost added to you current project; unless it's a space ship part. This is the only way to use production of one city for buildings in another city.
Freight has no other use.
Trade: You can trade with another civilisation using diplomacy. The other civ may refuse contact. Otherwise several trade options are available. Trading technologies makes the received advance and all further advances cheaper; and is thus very important for development. AI often will approve some trade deals, depending on whatever the programmer felt reasonable.
Posted 28-Feb-08 17:33 by Retudin