Trade Routes?

I cannot find much information on how trade routes work, are they the new way to trade luxuries? And can you choose what your trading in between cities, such as a set number of food from one city to another? And if for example, can you trade 6 food from city 1 to city 2, then from city 2 trade 6 food to city 3? And can you even trade food at all to other nations cities? Can somebody link a guide or explain how the entire system works?