Referring to links between a family of domains - for example your business site, your personal homepage and your cat's homepage. Cross linking is sometimes used to improve link popularity .However,
Google has updated its algorithm to identify clusters of sites which are strongly linked together but not well linked to the rest of the web, and applies a penalty to these sites.
Unfortunately , many innocent sites have been affected by this cross-linking penalty as it is a common practice to link to a corporation headquarter from every page of a subsidiary site.
What about sub-domains eg site1.domain.net and site2.domain.net --- Would that be considered the "same site" or would that be crosslinking?
Sub-domains are considered to be different sites. But I would be careful as to your linking structure between sub-domains, or as search engines call them, 3rd level domains. 3rd level domains, or subdomains, was one of the criteria that would have placed your site in the Ink Spam DB .