To figure out the difference of SharePoint Full vs. Incremental Search. Basically the incremental one only crawls content, the full one also indexes admin related stuff (metadata mapping). It is recommended to run a full crawl after updates and at least once a week. See full article here: http://blogs.technet.com/b/meamcs/archive/2013/02/18/sharepoint-2010-2013-search-incremental-crawl-vs-full-crawl.aspx
