Confused about trade goods
Can anyone explain trade goods to me? This isn't discussed in detail in the manual at all. I know that they have to be built like improvements on a planet, but what do they do for me after I get one? Also, how do I know the path to take that will end in me getting a trade good?
For instance, if I start a game as the Terrans, how do I know what research to follow to get to a trade good? Also, how many are there? Can all races get all of them?