This post is rather lengthy, but I think it is worth the space. It is an explanation of the GEDCOM format that we use so much. Although we use the term freely, I suspect, based on questions I hear at our classes, that the concept is not all that well understood.
This explanation is one that I think does the job of explaining it well. It was recently published in RootsWeb Review.
"If you have used genealogy software to create a family tree on your computer or you have created a tree online, you are probably aware that you can share your tree data with others who use a different software program because of a file called a GEDCOM (GEnealogical Data COMmunication).
"Genealogy software programs are databases and, as such, they arrange the information you input into fields. Each program does this using its own proprietary format. These formats are not compatible with one another. You wouldn't be able to share your trees with people who use other programs were it not for GEDCOMs. Understanding how the insides of a GEDCOM work will help you understand why your file shows up in each program the way it does.
"A GEDCOM is nothing more than a plain text file comprised of all the information you input into your genealogy file. You can open a GEDCOM in WordPad or any text editor. However, you may not be able to easily decipher the text when you attempt to read it in that manner. Think of the file content as being like an outline, where the indented lines explain the line above them. The numbers at the beginning of each line may be considered to be the number of indentations or tabs from the left of the page in a standard outline format. Thus a line beginning with the number 2 would contain details about the first line beginning with number 1 immediately above it.
"A GEDCOM uses "tags" to represent the fields in a genealogy database. Genealogy software programs support GEDCOMs by transferring the data in your file into tags. When you share a GEDCOM with someone using a different genealogy program than the one you use, the program uses the GEDOM tags to assign the data to the proper fields used by the alternate program. Some common tags are, SOUR for source, BIRT for birth, and PLAC for place."
[Previously published in RootsWeb Review: 8 September 2010, Vol. 13, No. 9]