Well, for example:
http://www.trannybumping.com
All the links to the galleries and other sites you see are going through my trade script. Only the few links below the Moviedollars banner at the bottom are going straight to the other sites. That are the so-called hardlinks. They're used for trading mostly PR/Search Engine visibility. Ofcourse some traffic gets traded too that way but not the same way/amount as the script trades.
Script trades: you get back what you send more or less
Hardlinks: you can send 50 visitors and get whatever traffic back.