Cross linking refers to the process of linking between two sites. Whether or not they are owned by the same person, cross linking is the process on which the internet is built. It allows users to reference sites with content similar to that which they are already viewing, and may be of further interest to them.
Cross linking also serves an interesting purpose in terms of search engine optimization. Link popularity is one of the main factors involved in how search engines determine the value, importance and relevance of sites on a given topic. This calculation is then reflected in the site's search ranking.
As a result, cross linking between sites is a very popular method of search engine marketing. Both reciprocal links and inbound links can be valuable SEO tools, provided you use cross linking effectively, especially if you are the owner of both web sites.