Hello again Linsys,
Link trades would usually be on your other types of sites other than free sites. Say for example a tgp or linklist that you build for yourself. You would trade links with like site in like spots.
Pr is a rating that google gives pages/domains. You can see the pr rating of anypage with the google toolbar that you can download
here if use ie. I think there is also one that you can use with firefox but not real sure.
The force that you read about is a thing when dealing with trade scripts. You need to have your script 'force' a certain amount of hits to the other persons site over a certain amount of time. Then with that incoming traffic their script will pick up on the trade and start trying to send traffic back.