Link exchange is an old concept and I wonder why paramita hasn't heard about it! Link exchange and back link (or link back?) are not the same, at least I think. In the former concept, people would visit your site and for this you also visit their sites. But in back link, you have to put your URL in some similar type of blogs or sites so that the visitors of those sites/blog may also visit your site by chance. May be I am wrong in understanding!