When you RECIEVE another civ's economic treaty, you recieve a bonus income equivalent to 10% of the weekly bc's produced by that civ. The civ that you recieved the treaty from does NOT lose any income. This continues as long as the treaty remains in effect - that usually means until you find yourself at war with the granting civ, or they get destroyed / assimilated.
When you RECIEVE another civ's research treaty, you recieve a research bonus equivalent to 10% of the weekly tp's produced by that civ. The civ that you recieved the treaty from does NOT lose any research points. This continues as long as the treaty remains in effect - that usually means until you find yourself at war with the granting civ, or they get destroyed / assimilated.
It is not necessary for these treaties to be mutual; you can make them part of any trade, using any tradable item/information as the other side of the deal.
Each civ, including you, is allowed to trade away 1 economic treaty and 1 research treaty at any given time. A civ may RECIEVE any number of treaties.
The AI tends to value economic treaties much more highly than research treaties, at least until late in the game. The AI also do not seem to trade research treaties much among themselves.
There have been times when, even playing a lab-heavy, research heavy strategy, I have recieved 25% or more of my total research points (check the top of the research screen) during the 1st 2/3s of the game from treaties. It is quite possible to collect ALL of the research treaties from all major and minor civs if you do it methodically as soon as you open trading with each civ, when they don't value their own research treaty very much. You'll probably have to trade 2-3 early techs for a major civ's research treay, though occasionally you can get the treaty + 1 trash tech for something they value disproportionately, like Trade. In my opinion, an early acquired r-treaty pays for itself within about 6 months.
In contrast, I can rarely get more than one or 2 econ treaties, usually with minors or heavy trading partners who don't have many other contacts, and I have to pay heavily. Trade and Advanced Trade are good techs to try an get an econ treaty in a deal. I consider it a good move to EXCHANGE econ treaties with my heaviest trading partner (if I give up my treaty at all), to solidify that relationship. Alternatively, I will give my econ treaty cheap to a minor or weak major that I am trying to shore up for awhile. It appears to me that the AI's tend to exchange econ treaties early with similar alignment civ's that they are allied with, or maybe that they are trying to ally with.
I NEVER give up my research treaty, except occasionally to a minor who I expect to be polished off soon anyway - but that is because I always expect to be the heaviest researcher in any game.
Hope this is useful.
drrider