Sources and CitationsA source is the origin of genealogical information that may become evidence after a genealogist has properly analyzed it as part of reaching a conclusion.
A source may be a document, book, website, person, artifact, photograph, audio recording, video recording, or other item containing data. Birth and Death Certificates, Wills, Contracts, and Census Records are all examples of sources.
Sources may be either original (not derived from a prior record) or derivative (extracted, transcribed into a typescripted or handscripted copy, abstracted, etc., from an original record).
A citation is a reference to a source, and is also known as a source note. A proper citation describes the location of the source in sufficient detail that a subsequent researcher can find the evidence. A proper citation also describes the specific details from the source that affected the analysis, and the specific place within the source where those details reside, such as a page number.
You should be careful to cite the sources you have seen for yourself. For example, in the case where a relative sends a letter and describes information taken from a Birth Certificate, the source is the letter, not the Birth Certificate. Such a letter should trigger a followup task to obtain a copy of the Birth Certificate.
In all cases, you should be careful to record whether you analyzed an original document or some form of copy (photocopy, microfilm, microfiche) or transcription (electronic database or web site, CD-ROM, etc.), and the source material the publisher used to make the copy or transcription.
There are many details involved in citing sources properly, many more than can be explained here. Evidence! Citation & Analysis for the Family Historian by Elizabeth Shown Mills explains genealogical standards for citation and analysis and is a must for every genealogist and family historian. Also see her article, Working With Historical Evidence: Genealogical Principals and Standards, NGSQ 87:3 (September 1999), published two years after her book, for a useful "Process Map for Analyzing Evidence" that further explains the terms cited here.