Unofficial Lore (variously known as Out-of-Game Lore, Forum Lore, Non-Canon Lore, or Obscure Texts) is content providing information on the world of Tamriel published not by Bethesda Softworks but by individual developers and writers, on forums or fan sites. This material is best represented by the list of Obscure Texts found at the Imperial Library.
Unofficial Lore takes the form of an in-character lore document and may be posted with several aims in mind. Some material is intended to be taken as a valid source or explanation of what was hitherto a mystery, and some is merely shared to encourage discussion among the fans (in other words exploring the importance a document would have if it were to be taken as canon). The material may have been cut or overlooked for a spot in the games themselves, or may have been written as an expansion of lore independent of any game. Some documents have served as teasers and previews of games yet to come. Several developers have published Unofficial Lore, primarily Michael Kirkbride, Gary Noonan, Douglas Goodall, and Ken Rolston. All except Noonan have since left Bethesda.
Some examples of Unofficial Lore include (but are certainly not limited to): The Hogithium Hall/Trial of Vivec Roleplay, The Love Letter from the 5th Era, 9th Era time travellers, The "Towers of the World" theory, the Thalmor conspiracy to destroy/deactivate said towers in order to destroy Mundus, the Aldmeri commentary on Talos involving killing man to re-ascend, etc.
Many doubt the validity of Unofficial Lore, while many loremasters do not distinguish between it and texts found in-game. It is useful in understanding many lore topics—in some cases it is integral to our knowledge of a particular area. However, it may represent the viewpoint of only one writer, as material published individually has not necessarily passed muster with the group of people who currently have creative control over the series. This may lead to a more personal understanding of lore that does not necessarily correlate with the series as it continues.
Bethesda has commented in the past that Unofficial Lore is not to be taken as official canon, as spoken by Gavin Carter, Todd Howard and Pete Hines during the Oblivion Third Fan Interview in 2006 when asked about the Hogithium hall roleplay, Fifth Era love letter, and other extra-canonical posts and information, commenting that "only things that have been published in Elder Scrolls games should be considered official lore". Note however that this statement pre-dates the two novels, The Infernal City and Lord of Souls, which have both been confirmed as official canon.
UESP tentatively allows Unofficial Lore to be used as footnote citations in lore articles, with caveats.