IGI is the International Genealogical Index and you can search it through the Mormon Website http://www.familysearch.org/Eng/Sear...set_search.asp
From a genealogical point of view you need to give most value to those entries which cite a published source such as parish records, for amongst these entries are many posted by members of the Church. These may be useful for guidance but remember that what you want are references to primary sources.