Reciprocal linking is a kind of 'symbiotic relationship' between two complementary websites. Each site has a hyperlink pointing to the other. The idea behind this is to elevate the awareness of the search engines' to the existence of both websites'. In Google's case, a lot of reciprocal linking could boost a site's page rank. Even better, reciprocal linking could raise the position of that website in Google's listings under specific keywords or phrases.