IGI stands for the International Genealogical Index, which is a partial index to vital records from around the world. It is one of the world’s largest collection of genealogical records. Information comes from individual research and original records. The majority of the names are from records of people who lived between 1500 and 1885. The index contains:
The names of millions of deceased persons from throughout the world.
Dates and places of births, christenings, marriages, and other events.
Latter-day Saint ordinance information.
Some of the information may have come from microfilm copies of the original records. Because the original records may have been updated by the record custodians, you may find additional, more accurate information in the original records. You may also be able to obtain official copies of the original records by contacting the appropriate record custodian.
The Church of Jesus Christ of Latter-day Saints (often referred to as the LDS) has made the International Genealogical Index (the "IGI") available on the internet at http://www.familysearch.org/ and provided a very effective searching capability. However, it is not always easy to locate your ancestors (even if they are there in the IGI) using the search mechanisms provided at that site. This is because a search by last name only is not permitted unless you search within a single batch of records at a time or, at minimum, across the entire country (not even a single county let alone a town). If you have a rare name this might be OK but what if you are looking for Smith or Jones?
There are the standard warnings about relying on the IGI - as with any secondary source it should only be used as a finding aid and the primary source material should be referred to for definitive information. There ARE mistakes but, nevertheless, used properly it is indispensible.
The LDS has organised the records in the IGI into 'batches'. These batches relate to how and when the information was extracted from source material. This is where Hugh Wallis comes to the rescue at his site http://freepages.genealogy.rootsweb.ancestry.com/~hughwallis/IGIBatchNumbers.htm , but more of that another time.