Link exchange is a way of providing link exchange systems and topic sites or partners with links for the purpose of increasing the index of quotation of your site. The quotation index directly depends on the number of links leading to your site from other resources.
At the link exchange you always need to remember that, the more popular is the site that refers to you (and the more his QI is) more points will be added to your citation rate index. The sites with little popularity and small QI will feebly influence the value of quotation index. There are many ways to «trick» the partner, starting with the link being removed some time later and ending with the very complex and difficult to detect ways of fraud in a link exchange. At the link exchange one should know that the search systems actively refine the filters which allow not to consider artificial links.