The ones that your civ does for you automatically, and that some wonders give you +2 routes in each city, etc? I cant find information on this from anywhere!

Any time your cities are connected by a trade network (roads, rivers, coast with Sailing or sea with Astronomy) it creates at least one Trade Route with a city it's connected to. It'll create the most lucrative route it can, which will usually be foriegn. These are a great source of income.

The link I've provided gives good information about how they are calculated and what they provide.

http://www.apolyton.net/forums/showthread.php?s=&threadid=148563