Reverse SEO is the practice of taking actions for decreasing the rank of a page or website in Google and other search engine pages. Reverse SEO may be used for penalyzing competition websites. It is then called negative SEO and the practice presents legal risks.
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.