This is not easy to give a short answer

The LDS church produced this index as a result of their beliefs. As I understand it, they submit records so that their ancestors can be baptised or accepted into their church even though they have long since died. This is done by:
1. submitting records extracted from the parish registers - by filming, transcribing and indexing the original entries;
2. submitting entires from their own research
The extracted entries are much more reliable since they come from the original registers themselves. However, there are still transcription errors and some countries or counties are covered better than others. The submitted entries are sometimes only "guesstimates." Ideally all entries should be checked. Use the IGI as guide.
PAF = Personal Ancestral File which is a genealogy programme, free to download from LDS site.
Ancestral File is a large database of pedigrees maintained by the Genealogical Society of Utah (closely connected to the LDS church) and included on the IGI.
Sometimes you can find out who submitted the details but if it was decades ago, it is not always helpful.
Nell