Link trading is the task of getting links to your site on other sites by trading.. umm.. links. This is done to get more traffic to you web site. Link trading creates more traffic in two main ways, increased search engine rankings and also from the traffic the links themselves generate from people clicking on them.
I guess you are referring to paid links exchange or may be free link exchange, but as such there does not exist anything as Trade Link Exchange! If you can ask with a bit of context, we can then go into more details about it!