As described by the archives: "In June 1996, the USGenWeb Digital Library (USGenWeb Archives) was developed to present actual transcriptions of public domain records on the Internet. This huge undertaking is the cooperative effort of volunteers who either have electronically formatted files on census records, marriage bonds, wills, and other public documents, or are willing to transcribe this information to contribute."
The Archives were originally held on Rootsweb's servers, but were moved to independent dedicated servers in 2008 (accordingly, some old URLs may not be correct).
The files are organized both geographically and thematically (births, deeds, etc.) for browsing. National and statewide search engines can be found here. Because of the nature of the project, there are often files about individuals extracted from larger sources, and also entire works. Before census images became more widely available, there seems to have been a concerted effort to post census transcriptions, particularly for 1850. There are also numerous cemeteries.
Individual records found in the archives that do not correspond to a printed source (i.e., cemetery transcriptions, letters, pension records) should be cited using "Citation Only" with a cite to the particular URL where they are found. Entire works posted in the archives should have the Archives listed as an available repository.