What is it?
The Ancestral File is a collection of user-submitted pedigrees submitted to the Church of Latter Day Saints from 1978 until several years ago (new submissions are no longer being accepted). Information on the "same" individual from multiple contributors is merged into the same entry, so it is the granddaddy of current projects like OneWorldTree and wiki projects like this one that seek to combine all the available information on a particular person. The database has over 40 million people in it, and can be a valuable resource representing the research of thousands of people.
The Ancestral File can be searched at using the links at left. You can narrow results using spouse, year of a particular event, state/country, or parents. You can search only for records with certain parents, as well. Records span virtually all surnames, locations, and time periods, but are for obvious reasons concentrated in the United States.
There are serious systemic weaknesses to the Ancestral File and pedigrees consisting solely of Ancestral File information should not be trusted.
First, the records are only as good as the submissions, and submitters were not ever held to a particularly high set of research standards or documentation. By itself, this fact, true of any information not listed with sources, requires the information to be treated with caution. But, as described below, the merging process virtually guarantees that bad data -- in particular connections between people who should not be connected -- is perpetuated.
Second, the merging process is, for lack of a better word, "dumb." It lacks a human check using common sense. Records will often have multiple spouses with the same name, impossible birth, marriage, or death dates, implausible jumps in time and place, or multiple sets of children born during the same time (the result of two people of the same name being improperly merged).
Third, there does not seem to have been an effective corrections process permitting people to be disconnected (removed from incorrect parents or children). If an old theory was rejected and the correct parents given in a new submission, the Ancestral File often just shows both sets of parents as "alternates." This means not only is bad original research included, but work that was published long ago and since proven incorrect lives on in the Ancestral File.
As a result, for any given record, the information may not be trustworthy due to errors at several points along the process. Because more recent (late 19th century and up) records will tend to have fewer submitters with more direct documentation and less need to merge, they are probably more reliable. The further back and the more submissions referencing a record, the greater the possibility for error.
It has been possible to download a gedcom directly from Ancestral File (either online or from CDs) for over 18 years. There are thus a number of pedigrees posted out there that consist of nothing but Ancestral File records. For the reasons stated above, these can't be trusted. Isolated citations to the Ancestral File need to be evaluated just like any other piece of information of unknown reliability.
Using Ancestral File as a Source
Consider information in the Ancestral File to be an unsourced clue and verify it elsewhere with a better source that is not some other online tree. Do not add the Ancestral File as a source to any WeRelate page if you can possibly avoid it.
Be aware of common errors:
These errors are common to online genealogy of all types, often related originally to the Ancestral File even if it is no longer listed as a source. If you see these errors on WeRelate, feel free to delete them. If contrary information appears in a well-respected source, it is fair to assume that the Ancestral File information is wrong and delete it. If you are undecided or unable after trying to confirm or deny the information, add your concerns/attempts to the talk page.
Just as with other online unsourced information, there are undoubtedly some instances where a careful researcher had access to family information that has not been published, such as a family bible. Although the original information may still not be available online or otherwise publicly accessible, use online discussion boards and check other trees submitted online to find relatives who can offer supporting information.
FamilySearch claims that pre-1500 European royal and noble families were "carefully scrutinized" before being added to the Ancestral File. Whether or not this is true, there are many other sources available for such families rather than citing the Ancestral File. If nothing else, most of these families should be in Wikipedia, which has the advantage of being much more current and has the ability (technically requirement) to cite sources.