|This article is about the current version of DF.|
If your fortress performs a successful Mission to "demand tribute", whether it be one-time or annual, that site will send a caravan, once a year. No actual "trading" or broker is involved, they simply drop off the goods at your depot and then leave, without speaking to any of your dwarves. Tribute caravans tend to be relatively small, but they are guarded. You don't have any control over the content of the tribute. What they bring depends on the civilization's available materials, the site's size and tracked items and so forth, and may range from excessively mundane (like a bunch of average quality clothing) to extremely useful (like exotic animals). Notably, tributes are one of the few ways to obtain evil animals tamed by goblins such as beak dogs for instance. Yearly tributes usually happen at the beginning of a season and may be arranged at any season, including winter.
If you successfully sent a squad on a mission to conquer a neighbouring site, it will also be incorporated into your holdings. Your conquering forces will remain onsite as administrators and occupiers, though you may call (most of) them back by requesting workers with your messenger. The site will also send tribute to you, in the form of various items and livestock.
Tributes are also one of the few ways to "contact" another civilization without triggering an outright war and therefore ensure that it will send out regular caravans afterwards, *on top* of the tribute caravans it'll be sending and even if your tribute demand failed.
You can receive tribute from any populated site, including necromancer towers. You can acquire tame sentients (including things that should just flat out not be tamable at all like goblins) from raids and demanding one time tribute.