Hi there Aussie Possom
The IGI is a computer compilation of various genealogical records originally created by the Church of Jesus Christ of the Latter Day Saints (Mormons) for the furtherance of their own religious ordinances.
During the process, immense reserves of information of use to all family historians were amassed, and by a considerable concession, these resources have been opened for the use of the general public.
The LDS filmed original parish registers/records from churches that would allow them access to their records, but, if a register was not available, Bishops Transcripts were filmed and used.
Most registers were either Baptism or Marriages because the intention of the LDS was to extend the membership of the church to the ancestors of their members, retrospectively, by collecting their details and presenting them in the Temple for baptism and sealing into church membership.
Very few of the people who transcribed the registers were genealogists, so many many errors occur in the transcriptions, plus members drew up their own family trees and submitted them to the temple - but a lot of them are incorrect- so beware of members own submissions.
The IGI is a fantastic finding aid, but you are well advised to re-check everything for yourself by viewing the original or film of the original.
Chris in 1066Land